Update link to contributors page

* GitHub changed their linking structure
Inline ActionMailerHelpers methods and perform cleanup

* remove Gemfile.lock from version control
* use secure connection to rubygems.org
* inline methods from ActionMailerHelpers into the only test that uses them
* alphabetically order some config lines
Simplify testing with different versions on Rails

* remove Appraisal's gemfiles directory
* don't specify gemfiles for Travis
* use pessimistic operator in Appraisal
* upgrade development gems, use pessimistic operator
Fix i18n keys for sessions