Enable unit test code coverage reporting

[lovell]

Hi Brandon, I was bitten by #197 and noticed that the accepted fix in #200 did not include a test that could have caught the problem.

I tried to generate the unit test code coverage report for this module with grunt coverage but received a "html-cov" reporter not found error.

It appears mocha dropped support for html-cov reporting in v3.0.0.

Given this module does not appear to require any of the features of Mocha v3, the quickest way to generate a coverage report was to downgrade Mocha to the most recent v2.x version.

With the small change in this PR it is now possible to use npm run coverage (or yarn coverage) to generate full code coverage reports.

Whilst "downgrading" Mocha may feel like a step backwards, the ability to generate a code coverage report provides a big step forwards in terms of confidence-building in this useful module.

If there's interest, I could take a look at modernising the test tooling to use something like nyc, which provides more useful statistics around branch coverage.

Cheers,
Lovell

[brandongoode]

@lovell -- Thanks for the offer to modernize the testing. I would gladly take you up on the offer. One of my other repos has already been moved over to istanbul and uses https://coveralls.io for coverage tracking.