fix: update rcov to simplecov

This commit replaces the :rcov task, which previously invoked rcov,
with the :coverage task, which invokes simplecov. Rcov does not work
with Ruby 1.9 and above.

Change-Id: If1188e7208e5a17cfafbf6b2154ffabbc6a0a0d8
Merge pull request #341 from infochimps-labs/master

merge github/master into gerrit/master

merging in already-reviewed changes and version bumps