Duplicate frozen value rubygems < 2 will modify

Was getting some errors about modifying frozen strings (seen [here](https://gist.github.com/RoboticCheese/8443728)) and tracked them down to [this](https://github.com/rubygems/rubygems/commit/48f1d869510dcd325d6566df7d0147a086905380#diff-94d386526b8b558035d4e4c7602d647c).

Pre-2.0 Rubygems tries to modify the argument passed to `Gem::Version.new`. I'll upgrade my system, but the fix was easy enough. Doesn't seem to be a good way to test this in Travis, though, without slowing the build down installing different versions.