fix(installer): hdiutil output should be a string

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

cross-spawn-promise returns stdout as a buffer.

@zeke could you see if this fixes your problem? I'm kind of flying blind since I don't have a Mac.

Fixes #410.

[zeke]

@malept thanks for working on this. I'm seeing a different error when attempting to install from a build of this branch:

~/git/electron-userland/electron-forge fix-dmg-install
$ node dist/electron-forge.js install SingletonIT/MUNO-release

✔ Checking your system
✔ Searching for Releases
Found latest release: v0.4.1
? Multiple potential assets found, please choose one from the list below: Muno-0.4.1.dmg
Downloading SingletonIT-MUNO-release-v0.4.1-Muno-0.4.1.dmg
[============================================>] 100.0% of 58.16 MB (202.47 kB/s)
✖ Installing Application

An unhandled error has occurred inside Forge:
A value [object Promise] was yielded that could not be treated as a promise

    See http://goo.gl/MqrFmX

From coroutine:
    at Promise.coroutine (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:194:17)
    at exports.mountImage (/Users/z/git/electron-userland/electron-forge/dist/util/hdiutil.js:75:39)
    at Object.<anonymous> (/Users/z/git/electron-userland/electron-forge/dist/util/hdiutil.js:90:3)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Module.require (module.js:517:17)
TypeError: A value [object Promise] was yielded that could not be treated as a promise

    See http://goo.gl/MqrFmX

From coroutine:
    at Promise.coroutine (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:194:17)
    at exports.mountImage (/Users/z/git/electron-userland/electron-forge/dist/util/hdiutil.js:75:39)
    at Object.<anonymous> (/Users/z/git/electron-userland/electron-forge/dist/util/hdiutil.js:90:3)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Module.require (module.js:517:17)
    at PromiseSpawn._continue (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:159:21)
    at PromiseSpawn._promiseFulfilled (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:99:10)
    at /Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:201:15
    at mountImage (/Users/z/git/electron-userland/electron-forge/dist/util/hdiutil.js:88:18)
    at Object.<anonymous> (/Users/z/git/electron-userland/electron-forge/dist/installers/darwin/dmg.js:37:51)
    at Generator.next (<anonymous>)
    at Generator.tryCatcher (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/util.js:16:23)
    at PromiseSpawn._promiseFulfilled (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/generators.js:97:49)
    at Promise._settlePromise (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/promise.js:574:26)
    at Promise._settlePromise0 (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/promise.js:614:10)
    at Promise._settlePromises (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/promise.js:693:18)
    at Async._drainQueue (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/async.js:133:16)
    at Async._drainQueues (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/async.js:143:10)
    at Immediate.Async.drainQueues (/Users/z/git/electron-userland/electron-forge/node_modules/bluebird/js/release/async.js:17:14)
    at runCallback (timers.js:781:20)
    at tryOnImmediate (timers.js:743:5)

[zeke]

Tried it with a few different repos that have DMGs and got the same result.

[malept]

Ah, I think I see the problem:

https://github.com/electron-userland/electron-forge/blob/fa0e3ea16bc73fc789689a96f894b9120c737e68/src/util/hdiutil.js#L29

The await spawnPromise(...) needs parentheses before the .toString(). I'll fix it tonight.

[malept]

@zeke I should have fixed the latest error, have another go at it when you have a chance.

[zeke]

Works!