core: Modify Bundler settings to work with restricted permissions

This includes two changes:

1. Change the Vagrant requirement to use the "= VERSION" constraint.
   The "path:" previously requires write permission in the directory
   of the gemspec, which we don't have in installers.

2. Set a BUNDLE_APP_CONFIG and BUNDLE_PATH setting. By setting this,
   Bundler won't ask for sudo password. Bundler never actually needs it;
   it is just overly aggressive in thinking it does.