[FIX] sourceRoot can sometimes be `undefined`

Caused `path.resolve` to complain with "Arguments to path.resolve must be strings Use --force to continue."
Added a regression "test" (rather a Grunt task) for undefined sourceRoot
fixed in bf5c36029b901d4da366d7fa4ced95c090cb0b3f
Merge pull request #4 from sirlantis/master

[FIX] sourceRoot can sometimes be `undefined`