Include FFC and PyOP2 version in FFCKernel cache key
Add PyOP2 / FFC version compatibility check
Merge pull request #225 from OP2/ffc-cache-key

Include FFC and PyOP2 version in FFCKernel cache key

This should make the FFC kernel disk cache more resilient to false
positives. Also adds PyOP2 / FFC version compatibility check.

Closes #195