use bundler compatible ruby version string

this fixes the invalid gem path with MRI. Bundler uses inconsistent version number schemes across Rubies: for example, JRuby 1.7.11 will have `1.9` in its gem path while MRI 2.1.0 will have `2.1.0`. in the #1437 refactor I wrongly assumed  the `major.minor` scheme was used across all Rubies.