Made it possible to use gulpfile.babel.js
Made it possible to use gulpfile.babel.js

When using Gulp through an ES6 Gulpfile, the filename is `gulpfile.babel.js` instead of `gulpfile.js`, resulting in the script failing to find the Gulpfile. This pull request intends to fix this.
timdp/heroku-buildpack-nodejs-gulp,celerityweb/heroku-buildpack-nodejs-gulp