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.
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