fix travis

1. .travis.yml's "rails-master.gemfile" has never worked. This is because the file's named "rails_master.gemfile".

2. Nokogiri 1.6 and Timecop 0.6.2 require Ruby > 1.8.7, which unsurprisingly breaks all our 1.8.7-compatible builds. So I'm specifying compatible versions in the gemspec. If you guys think we should put horribleness like that in the rails-3-2 gemfile instead, I'm happy to move it.