call done() callback with success boolean

In an older project I updated `grunt` to `v0.4.2` and `grunt-mocha`  but then the task (and so my build) doesn't fail when the tests fail:

    ...    
    >> 3/6 tests failed (0.13s)
    
    Done, without errors.

It is fixed by adding the status booleans to the done() call.

    ...    
    >> 3/6 tests failed (0.12s)
    Warning: Task "mocha:base" failed. Use --force to continue.
    
    Aborted due to warnings.

It looks like `grunt` wasn't notified of failure by the log-features somehow. Adding the booleans makes the status more explicit, as per http://gruntjs.com/api/inside-tasks#this.async