Fix Assets on Deploy

Precompiles assets (read: less stylesheets) on deploy. Also fixes useminPrepare to **not** run the "concat" task for css (it's not needed since the "cssmin" task concats files itself). This allows us to properly deploy image assets referenced by css in our bower components.

Fixes https://www.pivotaltracker.com/s/projects/957886/stories/64399706.