Ditch babel5 dependency

[blowery]

Pulling a hard dependency on babel 5 is unnecessary. Consider switching to es6-promise which doesn't have as much baggage for downstream projects.

[TooTallNate]

@blowery babel-runtime is used in other places as well unfortunately, so it's not just Promise.

I'm thinking that upgrading to Babel 6 and a major version bump for wpcom.js might be good enough to satisfy Automattic/wp-calypso#1515. Yay or nay?

[TooTallNate]

Note that I'm also planning on removing babel as a dependency in general. It should be a dev dependency and not installed when being consumed by a parent module.

babel-runtime as a dep I don't really see any way of removing, short of refactoring all the code to not use ES6 anymore, but I don't think that's the direction we want to go in.

[TooTallNate]

Fixed by #201.