Use gem platform to get specs

Using platform-dependent gems I was experiencing some issues getting their dependencies and generating the atom feed. These changes should fix those issues.

By the way, trying to write a test for the atom feed I saw `inject_gems` (`test/test_helper.rb`) is not working as expected for me, not even with the existent test. Instead of including the gem you are injecting, it ends up indexing some other gems but not the one you want. I couldn't fix the problem so far, but I think it's better to make the pull request now anyway.

Cheers!