Fixing grunt-concat options for wrapping the build code in an closure

I just fixed the using of the banner/footer properties on grunt-concat task for wrapping up the distribution file inside a closure.

[Here is a reference of grunt-concat options implementation](https://github.com/gruntjs/grunt-contrib-concat#concatenating-with-a-custom-separator) from their docs, which I followed to fix this issue.

I'm just submitting the fix on the `gruntfile.js` to keep it an atomic PR, bear in mind a new release of the distribution files will be required to provide the fix for final users.

fixes #109