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

src/lib/type.js

@@ -1,9 +1,10 @@
-const SemanticReleaseError = require('@semantic-release/error')
+var SemanticReleaseError = require('@semantic-release/error')
 
 module.exports = function (config, cb) {
-  const { plugins, lastRelease } = config
+  var plugins = config.plugins
+  var lastRelease = config.lastRelease
 
-  plugins.analyzeCommits(config, (err, type) => {
+  plugins.analyzeCommits(config, function (err, type) {
     if (err) return cb(err)
 
     if (!type) {