fall back to _$jscoverage

Hi @alex-seville.

I was kicking myself trying to figure out why the test I wrote for my blanket changes was failing until I realized it was the reporter... so before I can make a passing test for my blanket changes, I had to do this.

If window._$blanket is not defined, fall back to window._$jscoverage.

This allows blanket coverage for files that are pre-instrumented with grunt-blanket.