Automate Erlang builds

• Compilation of Erlang versions are now automated by TravisCI builds
• To support a new Erlang version, send a PR to https://github.com/HashNuke/heroku-buildpack-elixir-otp-builds by adding the latest erlang version on the first line of the otp-versions file
• Switched to a dedicated source for Erlang builds

Lots of changes and a stable buildpack

• Major changes since v1
• Pre-compiled Erlang builds from hex.pm courtesy of @ericmj
• Lots of config options

Initial version of the buildpack

Supports Elixir v0.x.y (really old versions)