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

[christhomson]

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. :)

[naiyt]

Works for me in Rails 3 and 4, thanks for the patch @christhomson.

[cantino]

Plans to merge this?