Fixed issue 3285

When using prerelease gems we need to ensure we store the gem version otherwise Bundler won't use the correct gem version since prerelease gems are not available by default. By storing the gem version for prerelease gems, we allow Bundler to properly resolve prerelease gems and not give potentially confusing errors to Vagrant users using prerelease plugins.
Merge pull request #3301 from mitchellh/fix-issue-3285

core: fix prerelease gem versions