Actually shut up STDOUT during testing

In the spirit of issue #3, this removes the "calculating metric: <calculator name>" output when running rake.

However, I'm still getting a message "sh: git: No such file or directory", and I'm having a hard time figuring out where it's coming from, because the specs appear to have side effects.  The output appears to be coming from spec/turbulence/turbulence_spec.rb; however, the code explodes when I run that spec file by itself.