Remove simplecov

simplecov 0.9 dropped ruby 1.8 support, and stdlib is one of the oddball
modules that uses it. So we could probably just remove it and be okay.

(cherry picked from commit a7c129b22d91fc723a8176c066a3eb96b03a2f56)
Merge pull request #322 from hunner/remove_simplecov

Remove simplecov