Remove the action_mailer gem dependency

[metavida]

We'd like to use the sms_fu gem with a non-Rails app using Pony, but the way you have dependencies set up, we're still required to install the action_mailer gem. I may be reading the code wrong, but it looks like the action_mailer-dependent SMSNotifier is only really used in non-pony mode. Instead of a gem-requirement, how about just raising an error when an SMSFu::Client is configured to use :action_mailer and it's not installed?