Disable coverage for now

This is to make tests run. Currently we are not using coverage reports,
and a recent fix to remove development dependencies from the normal
installation has caused coverage dependencies to be removed and thus
fail.

We will revert this once we figure out how to make tests run with
separate bundler groups.
Merge pull request #1376 from jordansissel/fix/test-failure-due-to-coverage-missing

Disable coverage for now