This pull request adds an enhancement to the internal Bundler class
to cache solution sets. This prevents Vagrant from generating a
solution for configured plugins on every run. Modifications to
the configured plugin list (global or local) will result in the
cached solution being invalidatd and resolved again.
Also included is the removal of the GEMRC modifications required
for Windows.
In recent Rubies the first dependency to satisfy the constraint will
be used regardless if higher versions are available in subsequent
sources. Move custom source to start of list when resolving plugins
to provide desired behavior.
