Refactor metrics:coverage task to be simpler
Refactor metrics:coverage task to be simpler

* There is no need to test for simplecov inside the task. The gem is
  always installed and it is a noop when it is not usable.

JonathonMA/devtools,mvz/devtools,mbj/devtools,zaidan/devtools,sferik/devtools,backus/devtools