Rename Rubygem(s) to gem(s) in pagination

In both the gem index and search, the text "Displaying all x Rubygems"
is now "Displaying all x gems". will_paginate pluralizes it when
appropriate.

Also added assertions to existing tests in the rubygems and searches
controller test cases.
Merge pull request #529 from joonty/better_paginated_name

Rename Rubygem(s) to gem(s) in pagination