watch.scripts.files won't work with `**/*.js`

[paolodm]

grunt-contrib-watch would not work with the following configuration

            grunt.config.set('watch', {
                scripts: {
                    files: '**/*.js',
                    options: {
                        spawn: true
                    }
                }
            });

but it did with this:

            grunt.config.set('watch', {
                scripts: {
                    // notice I specified the directory here
                    files: 'directory/**/*.js',
                    options: {
                        spawn: true
                    }
                }
            });