Try adding Coveralls.io support.

It has to be turned on at coveralls.io before this can work.

* Using "dzil cover" instead of "dzil test" because it supports running cover commands inside the build directory where the cover_db lives.  Could maybe do it with HARNESS_PERL_SWITCHES.
* Because "dzil cover" doesn't support test switches, RELEASE_TESTING=1 simulates "dzil test --release"
* Put all the cpanm installs into one line for efficiency.
Merge pull request #166 from plu/feature/coveralls

Try adding Coveralls.io support.