Introduces sentry, removes airbrake

Sentry is the premiere FOSS error tracking app.

config/secrets.yml.example

@@ -31,8 +31,8 @@ production:
   # Generate your own with rake secret or use the environment
   # secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
 
-  # Your errbit API key
-  # errbit_key: '12345'
+  # Your sentry.io DSN key
+  # sentry_dsn: '12345'
 
   ########## OMNIAUTH Providers ##########
   # Leave the variables' names empty, unless you use the providers, in which