Introduce new switch for paver run_quality command.

When using -p, the underlying diff-quality command will pass that
number as a value for --fail-under.

For example,
  paver run_quality -p 75

Will pass 75 into the --fail-under switch on diff-quality.

Also upgrades diff-cover in order to take advantage of this funcitonality.
Merge pull request #5552 from edx/benp/diff-cover-threshold2

Benp/diff cover threshold2