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/verify.js

@@ -1,9 +1,12 @@
-const parseSlug = require('parse-github-repo-url')
+var parseSlug = require('parse-github-repo-url')
 
-const SemanticReleaseError = require('@semantic-release/error')
+var SemanticReleaseError = require('@semantic-release/error')
 
-module.exports = function ({pkg, options, env}) {
-  let errors = []
+module.exports = function (config) {
+  var pkg = config.pkg
+  var options = config.options
+  var env = config.env
+  var errors = []
 
   if (!pkg.name) {
     errors.push(new SemanticReleaseError(