chore: remove babel, fix integration tests

This commit removes babel/es6 from all source and test files, because it was introducing a lot of overhead and only little gain.
This commit fixes and enables integration tests on Travis.
This commit fixes #153 and #151 along the way.

_Originally this commit should have only removed babel, but without working tests that's a bit too hairy._
_I only realized that half way into removing babel/es6, so things are all over the place now._

Closes #153, Closes #151

test/mocks/child-process.js

@@ -4,10 +4,10 @@ const rawCommits = [
 ]
 
 module.exports = {
-  exec: (command, cb) => {
+  exec: function (command, cb) {
     if (/contains/.test(command)) {
       if (/notinhistory/.test(command)) return cb(new Error())
-      return cb(null, `whatever\nmaster\n`)
+      return cb(null, 'whatever\nmaster\n')
     }
 
     cb(

test/mocks/github.js

@@ -1,6 +1,12 @@
-module.exports = () => ({
-  authenticate: () => true,
-  releases: {
-    createRelease: (release, cb) => cb(null)
+module.exports = function () {
+  return {
+    authenticate: function () {
+      return true
+    },
+    releases: {
+      createRelease: function (release, cb) {
+        cb(null)
+      }
+    }
   }
-})
+}