Use simplecov instead of rcov.

This PR replaces rcov with simplecov, which supports ruby 2, as well as 1.9.3 and 1.8.7.

Tested with (bundle exec) `rake coverage` on ruby 2.1.2 and 1.9.3.

Set ENV['COVERAGE'] from the :coverage task, for backwards compatibility, though preferred way to run this would just be `COVERAGE=true bundle exec rake test`.

I wasn't able to get rcov running on master, so I couldn't compare the output of simplecov with the (old) rcov output, but the simplecov output seems reasonable.