Replace calls to Proc#bind (deprecated in Rails 4).

Rails 4 complains with the way `bind` is being used in Fixjour: `DEPRECATION WARNING: Proc#bind is deprecated and will be removed in future versions.`

I've changed it to use `instance_exec` if available, or `bind` otherwise.

Unfortunately, I couldn't get an old version of rspec working in my development environment in order to run the tests. I did a quick check and It Works For Me™ on both Rails 3 and Rails 4. But the tests should be run before merging of course. :)