spree_frontend.css.scss need precompiling

I noticed ```spree/frontend/spree_frontend.css.scss``` is not on precompile list so it doesn't work after a deploy.
Instead of putting it on the list i changed the name to reflect the default ```spree_frontend``` one.
This way seems easier to me, i can plug in this extension without need to change nothing, and i'm still using my app's ```vendor/assets/stylesheets/spree/frontend/all.css```.
What do you think?