Small tweak to package.json makes 'npm test' work again in Windows

Windows doesn't handle dot slash notation well, so I tweaked the `test` script in `package.json`. I'm happy to report that all tests pass in Windows!

    $ npm test

    > grunt-cli@0.1.7 test c:\Documents and Settings\apenneba\Desktop\src\grunt-cli
    > node bin/grunt test

    Running "jshint:all" (jshint) task
    >> 5 files lint free.