Add source code instrumentation support for the 'blanket' coverage tool

Hello,

I've added support for Alex Seville's `blanket` code coverage library, very much in the vein of the existing `istanbul` code coverage support.

There's a fairly simple unit test that verifies the state of the `baz` test fixture source code both before and after adding instrumentation.

I also removed Node.js versions 0.6 and earlier from the Travis CI job as these are no longer supported - https://github.com/npm/npm/issues/4379 provides more details.

Cheers,
Lovell