Without this change, the version string may be mutated when passed through
Gem::Version.new(version).to_s before being used to construct the box
directory. This is a problem when using prerelease version numbers because,
for example, it will look for "~/.vagrant.d/boxes/1.0.0.pre.alpha.1" when it
should be looking for "~/.vagrant.d/boxes/1.0.0-alpha.1".