Resolve bug with execAsync

Hi there!

We (@fabien0102 and I) faced basically the same issue than #1 and have a lot of troubles in loading `execAsync` module -which wasn't defined as a dependency-.

Finally, we found that the `image-size` module was much more adapted to do the job it's supposed too.

According to `grunt test`, nothing seems to be broken and it resolve the issue for us.
Hope that could help. Cheers!