[Fix] [Travis] Execute 'go vet', use stages, use language conf list, add go 1.12.x, update shellcheck

.travis.yml

@@ -1,22 +1,30 @@
 language: go
 
+stages:
+  - diff
+  - test
+
+go:
+  - 1.10.x
+  - 1.11.x
+  - 1.12.x
+  - tip
+
 matrix:
-  include:
-    - go: 1.10.x
-    - go: 1.11.x
-    - go: tip
-    - go: 1.11.x
-      script: diff -u <(echo -n) <(gofmt -d -s .)
   allow_failures:
     - go: tip
+  include:
+    - stage: diff
+      go: 1.12.x
+      script: diff -u <(echo -n) <(gofmt -d -s .)
 
 before_install:
   - mkdir -p bin
-  - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.4.3/shellcheck
+  - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.6.0/shellcheck
   - chmod +x bin/shellcheck
 script:
   - PATH=$PATH:$PWD/bin go test -v ./...
   - go build
   - if [ -z $NOVET ]; then
-      diff -u <(echo -n) <(go tool vet . 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');
+      diff -u <(echo -n) <(go vet . 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');
     fi