Updated Rails from 4.1.4 to 4.1.5

This fixes a vulnerability in ActiveRecord that bypassed strong parameters. I don't think this app was affected but better safe than sorry.

About this release:

http://weblog.rubyonrails.org/2014/8/18/Rails_4_0_9_and_4_1_5_have_been_released/

about the vulnerability:

https://groups.google.com/forum/#!msg/rubyonrails-security/M4chq5Sb540/CC1Fh0Y_NWwJ
Updated brakeman gem 2.6.1 -> 2.6.2

Among other things this new version checks if the vulnerability fixed by Rails 4.1.5 is present.
Updated Devise gem 3.2.4 -> 3.3.0

Also changed i18n strings from "sign in" to "log in", in line with the defaults in the new Devise version.

See:

http://blog.railsapps.org/post/94860442635/railsapps-update-for-devise-3-3
Updated aws-sdk gem 1.50.0 -> 1.51.0
Updated cancancan gem 1.9.1 -> 1.9.2
Removed explicit dependency on sax-machine from Gemfile.

This app only needs sax-machine because Feedjira requires it. If it ever becomes an optional dependency, I'd like to be able to remove it.
Updated capybara 2.3.0 -> 2.4.1 and capybara-webkit 1.2.0 -> 1.3.0
Updated coveralls gem 0.7.0 -> 0.7.1
Updated capistrano-bundler 1.1.2 -> 1.1.3
Updated rspec-core gem 3.0.3 -> 3.0.4
Updated rspec-expectations gem 3.0.3 -> 3.0.4
Updated rspec-mocks gem 3.0.3 -> 3.0.4
Updated tests to account for the "Sign in" -> "Log in" change.