instrument: make the task less verbose by default
instrument: make the task less verbose by default

Right now, running 'grunt instrument' will output something like

  $ grunt instrument
  Running "instrument" task
  OK
  OK
  OK
  OK

  Done, without errors.

The repeated 'OK' lines seem unnecessary and uninformative to me. They
now only appear when Grunt is run with --verbose.

acvetkov/grunt-istanbul,MailOnline/grunt-istanbul,taichi/grunt-istanbul