fix(PngCompressor): for #12

This will fix #12.
I'm facing the same issue and I found this is png image error. In my case, it's [admin_notes_icon](https://github.com/gregbell/active_admin/blob/master/app/assets/images/active_admin/admin_notes_icon.png).
I tried to compile that file using cli and it will error, so here's a quick fix.

* don't delete in_file
* catch exception during compression