updated to match spree >= 2.1 branch better (rails4 changes), switched to using deface for at least the thumbnails

The build was broken when using with >= spree 2-1-stable and rails4/ruby2 -- was referencing things that weren't defined and expecting nil. 

For the _image partial I pulled in the proper conditional from spree_frontend.

For the _thumbnails partial I made a deface override to reduce codebase and allow us to keep in line with spree_frontend easier.

Seems like deface should be used for the _image partial too but it still seems more elegant to rewrite it at this point, since the changes are so significant and the original file so minimal.