One more change to be grunt-0.4.0 compatible

When using grunt-qunit-cov-0.3.0 with grunt-0.4.0 I get the following error:

    Fatal error: Object #<Object> has no method 'expandFileURLs'

This pull-request attempts to fix this, following the advice on https://github.com/gruntjs/grunt/wiki/Upgrading-from-0.3-to-0.4.