switch mail gem to 2-7-stable git branch to fix https://github.com/mikel/mail/issues/1238

Gemfile

@@ -13,7 +13,7 @@ gem 'hashie'
 gem 'authie', '~> 3.0'
 gem 'dynamic_form'
 gem 'changey'
-gem 'mail'
+gem 'mail', :git => 'https://github.com/mikel/mail.git', :branch => '2-7-stable'
 gem 'autoprefixer-rails'
 gem 'bunny', '~> 2.5.1'
 gem 'secure_headers'