Replace Foreman::Env with dotenv

This replaces `Foreman::Env` with dotenv's implementation. All the specs pass.

[dotenv](https://github.com/bkeepers/dotenv) was originally an extraction of foreman's `.env` code into a separate gem to fix issues with running a rails app outside of foreman (like `rails console` and rake tasks). It has gotten more popular lately, and it has been a bit of a challenge to maintain compatibility with Foreman. 

dotenv 0.7 is not released yet, but if you're happy with this approach and nothing else needs changed in dotenv, I can push it out.