Test against modern rubies.

Test against 2.0.0 and 2.1.1. Scratch 1.8.x. Keep 1.9.3 for people
lagging behind on upgrading, but allow the build to fail on it.
Remove incompatible rcov

rcov is not compatible with ruby 2.x
add coveralls
Merge pull request #46 from tamird/simplecov

Coveralls