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/get-registry.js

@@ -3,8 +3,8 @@ module.exports = function (pkg, conf) {
 
   if (pkg.name[0] !== '@') return conf.get('registry') || 'https://registry.npmjs.org/'
 
-  const scope = pkg.name.split('/')[0]
-  const scopedRegistry = conf.get(`${scope}/registry`)
+  var scope = pkg.name.split('/')[0]
+  var scopedRegistry = conf.get(scope + '/registry')
 
   if (scopedRegistry) return scopedRegistry