Drop the Ruby 2.0 requirement in the Gemspec.

This brings back support for running this gem on Ruby 1.9. The gem
works fine on both 1.9 and 2.0. Unfortunatly with the ~> constraint
operator it is not possible to specify both ~> 1.9.3 and ~> 2.0, so the
>= 1.9.3 constraint is used instead.
Merge branch 'master' into one-nine

Conflicts:
	github-pages.gemspec
Merge pull request #18 from github/one-nine

Drop the Ruby 2.0 Requirement in the Gemspec