Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

lib/sms_api.rb

@@ -8,6 +8,7 @@ class SMSApi
 
   def url
     return "" unless end_point_available?
+
     open(Rails.application.secrets.sms_end_point).base_uri.to_s
   end