Set default LANG to en_US.UTF-8

Sets en_US.UTF-8 following the pattern used by the Ruby and Python
buildpacks. Essentially, configure LANG default for the build, and
in the profile which creates the default context for the running
application.

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