fix 2 more places we were using incorrect css3 gradients

these 2 were missed in: g/35808

fixes: #CNVS-13294
firefox can render gradients prefix-free but
all of these were using an invalid syntax for css3
gradients. The invalid syntax still works in chrome
but not firefox (or other browsers).


test plan:
make sure these look right:
1. Assignments Gradebook:Grid View 
   http://screencast.com/t/eVop4gcbogm
2. Assignments Gradebook:Grid View Settings Dropdown 
   http://cl.ly/image/1g0M203Z250c 
   (BG color on hover isn't showing)
3. SpeedGrader student dropdown 
   http://cl.ly/image/2P2g0O3I3R08

Change-Id: I35b56ae99844504b5205034138e56911653ecc53
Reviewed-on: https://gerrit.instructure.com/36215
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>