don't boolify string param "sourceRoot"

By default gulp-coffee specify sources in the sourcemap as filename, not as path. So, you cannot debug in IDEA or Chrome.

My gulp task:
```javascript
gulp.task('compileTestData', function () {
  var sourceRoot = 'testData/coffee';
  gulp.src(sourceRoot + '/*.coffee')
      .pipe(changed(testDataCoffeeOut, {extension: '.js'}))
      .pipe(coffee({bare: true, sourceMap: true, sourceRoot: path.resolve(sourceRoot)}))
      .pipe(gulp.dest(testDataCoffeeOut))
});
```

Easy solution exists — just specify sourceRoot. But gulp-coffee boolify this option value, so, "/Users/develar/Documents/idea/plugins/JavaScriptDebugger/testApp/testData/coffee" converted to "true".