Bumped the version required as the last commit uses a function that was introduced in 2.6
CONTRIB-4960 - Ensure grade item exists to avoid 'Invalid argument supplied for foreach()' error

Thanks to David Balch for reporting this and providing a patch.
CONTRIB-4961 - Allow certificate types to specify if they want the grade to have a prefix or not

Thanks to David Balch for reporting this and providing a patch.
Increased version number
Replaced $module with $plugin in version.php
Updated version required