Fix a bug in diff-cover

This updates `diff-cover` to version 0.1.2, which fixes a bug in which missing lines were over-reported.

Here are the changes in the `diff-cover` repo: https://github.com/edx/diff-cover/pull/8
(So far, I've been merging changes in the `diff-cover` repo myself so I can iterate quickly and get the tool integrated into code review.  From this point onward, PR's in the `diff-cover` repo will follow the normal code review process.)

Suggested reviewers: @cpennington @brianhw