Only run our minitest plugin code if SimpleCov is indeed defined

[PragTob]

• Fixes Problem with loading Minitest simplecov plugin #877
• not quire sure whether we should wrap the entire thing in
  if undefined?(..) or not but since the file has all the right
  structure otherwise I'm more worried about minitest being
  weirded out that it doesn't find what it expects than I am
  of letting it execute an empty method
• also yes the version workaround is maybe a bit weird, however
  I wanted our test suite to reproduce the exact failure case
  because otherwise it'd be to easy to just implement a
  respond_to? check which would have everything passing but would
  fail in real life.