Be quiet about coverage locally.

The yellow on every test run is causing me to have minor panics every
time when I wonder what I broke and/or made pending. This shuts off
coverage by default. It can be optionally re-enabled with the COVERAGE
environment variable, so that you can still do:

COVERAGE=true rspec spec

If you'd like to see the current behavior.
Merge pull request #430 from CenturyLinkLabs/chore/less-chatty-coverage

Be quiet about coverage locally.