Run as non-root user

Generate a random user on each deploy and run the compile step as well as the final application with this restricted account. This should solve the problems with the permissions in the PHP buildpack and the Bundler warning in the ruby buildpack.

References:
  * #88 - security?
  * #98 - Run apps as non-root user, take 2
  * #42 - Allow apps to run without root
Update test apps for latest setup