fix rounded hover state on module cog dropdown
fix rounded hover state on module cog dropdown

Fixes: CNVS-29485

test plan:
- as a teacher - navigate to a course that has modules setup (if
  you do not have a course with a module - create one)
- ensure the module has a few assignments or quizzes in it
- select the cog that appears to the right of the assignment
  name (where you have the option to increase indent, edit, move
  to, etc...) the hover state on the first item should no longer
  be rounded (old view: http://screencast.com/t/itbP0KxCz2ze)
  new hover view (http://screencast.com/t/wKqmXYUHjy)

Change-Id: I400d63cef886238db47425500f44c5862ce78b77
Reviewed-on: https://gerrit.instructure.com/80147
Tested-by: Jenkins
Reviewed-by: Chris Hart <b82b40b84a2f06957ec9caf1768340bd6b929cc4@instructure.com>
Product-Review: Blake Simkins <b120bacfd01287a1de7e8077d020b48e9650385b@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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