Improve error handling if can't write to cache

 * Extra calculate/write cache into method
Get rights working on rails 4.1
Specify primary_key so can do rights_roles as has many through
Use latest dependent restrict when testing
Add has_privileges_of? method

 * Allows you to know if a user has all the privileges of another
   Useful if you want to allow users to login as lesser privileged users
Drop ruby 1.8/1.9.2 support

 * 1.8 isn't supported anymore and 1.9.3 support remains
 * Rather than having to lock to older version of rest-client for testing