Update gems

- Bump rails to 4.1.7. See [release announcement on the blog](http://weblog.rubyonrails.org/2014/10/30/Rails_3_2_20_4_0_11_4_1_7_and_4_2_0_beta3_have_been_released/).
- Bump coffee-rails to 4.1.0
- Bump rspec-rails to 3.1.0
- Bump factory_girl_rails to 4.5.0
- Bump guard to 2.7.0
- Bump rubocop to 0.27.0