Replace Solve with RubyGems in Vagrant version checking

`Solve::Constraint` doesn't support pre-release version numbers[1], so
replace it with `Gem` classes while waiting for a fix.

Fixes #97

[1]: https://github.com/RiotGames/solve/issues/29
Merge pull request #100 from tmatilai/vagrant-version-check

Replace Solve with RubyGems in Vagrant version checking