Simplify rubygems version string.

Rubygems only supports letters, numbers, and period as part of the
version string. This uses the new structure implemented in
https://github.com/emberjs/ember.js/commit/3d8d23af6d9f1387ee37d9588bd567704730ee21
and simplifies the generated rubygems_version_string for non-published
versions.
Merge pull request #3605 from rjackson/follow_rubygems_version_rules

Simplify rubygems version string.