This fixes a minor styling issue where all items in the account navigation
have rounded borders (on the bottom left and bottom right) in the hover
state, when only the last item needs this styling applied.

Verified on Chrome, Safari, Firefox (all latest). Did not test in IE (no access to Windows at the moment). :last-child isn't supported for IE < 9, but neither is border-radius, so it shouldn't affect older versions of IE.