Overwrite environment variables passed to Emacs

This will overwrite the environment variables passed to Emacs using the
users default shell if supported.

This is along the same lines as pull request https://github.com/caldwell/build-emacs/pull/30.  It should address the concerns you had with pull request https://github.com/caldwell/build-emacs/pull/30 and https://github.com/filsinger/build-emacs/commit/38e7eb1b36a2c627e639f5ea1a27bfbbb7e5ad4a.   

I've done some basic testing on Yosemite with ruby "1.8.7" and "2.0.0" via the command line (I couldn't launch via finder after replacing the launch script inside the Emacs.app due to code signing).