Updating mysql2

The new version has many improvements on compatibility and memory
allocation. It also have a few new options for connection.
Updating devise

This fixes CSRF token fixation security issue.
Updating rspec-rails

Performance and compatibility fixes
Updating prawn

This update comes with huge new features and much better memory
handling.
Updating cancan

Some fix and security improvements
Merge branch 'gem_update' into 4.0.x
Merge branch '4.0.x'