Update grunt-karma.js

[enigmak]

Fix for bug: arguments to path.resolve must be strings #142

I changed the grunt-karma.js file by adding data.files =_.flatten(data.files); to line 85. The _.flatten used to be there in the 0.9 version and the [].concat is not flattening like it should on my machine.

To test it pass files: [['a.js','b.js','c.js'],'d.js','e.js'] under karma -> options -> files in the config and it will break then add the line of code and it will work.

[dignifiedquire]

Thanks, could you change the commit message to something following http://karma-runner.github.io/0.12/dev/git-commit-msg.html e.g.

fix: Flatten files array.

Closes #142