Set default LANG to en_US.UTF-8

Sets en_US.UTF-8 as a default in the buildpack following the pattern used by the Heroku-supported Ruby and Python buildpacks. Essentially, configure LANG default for the build of ghc itself, and in the profile which creates the default context for the running application.

This commit changes the previous approach of using `/bin/release` for setting configuration variables. According to the accepted answer for this SO question, and based on the supported Ruby and Python buildpacks, we should be doing this through the [profile.d script](https://devcenter.heroku.com/articles/profiled) that is sourced by Heroku.

References:

* Python buildpack language encoding:  https://github.com/heroku/heroku-buildpack-python/blob/566f7f4555dcde26252a0db01f6bd28f6bb00494/bin/compile#L84
* Ruby buildpack language encoding: https://github.com/heroku/heroku-buildpack-ruby/blob/2eb97fcbf5e369800596ba6b070fde0ef17c59e9/lib/language_pack/ruby.rb#L60
* SO question about why Heroku doesn't set a default UTF-8 LANG: http://stackoverflow.com/questions/13799636/why-changing-heroku-buildpack-for-existing-app-doesnt-run-bin-release