Update p5-Devel-Cover from version 0.67 to version 0.68.

Upstream changes:
Release 0.68 - 5th August 2010
- Fix gcov2perl to work with large numbers (Thomas Dorner) (rt 45028).
- Fix "gcov -l" include files (Thomas Dorner) (rt 44864).
- Test against 5.12.1.
- Gross workaround for for regexp evals (Florian Ragwitz).

(seb)