Work around for OSX 10.10 PATH issues

This adds the `env=os.environ` option to the `subprocess.Popen` call.  It provides a decent work around for the upstream PATH problems in OS X 10.10.  

Work around for https://github.com/kemayo/sublime-text-git/issues/368

See https://github.com/SublimeText/LaTeXTools/issues/401#issuecomment-59649977 for details on how this fix works.
Same issue here: https://github.com/SublimeGit/SublimeGit/issues/150