DLTP-274: Fix link hover style for IE11

The link style was transitioning between transparent and a dark color.
Now it is transitioning between two colors.

Normal browser behavior is to show the background color from the
containing element. IE 11 was showing the background image for the hero
image instead.

Closes #274
Merge pull request #163 from ndlib/DLTP-274

DLTP-274: Fix link hover style for IE11