Fix "Phabricator" logo hover state in Firefox

Summary:
For some reason, Webkit parses the completely made-up "background-position-y"
property. Firefox does not. Use a real property instead of a creative one that
doesn't exist.

Test Plan:
Hovered over "Phabricator" logo in Firefox, Safari.

Reviewed By: codeblock
Reviewers: codeblock, aran, jungejason, tuomaspelkonen
CC: aran, codeblock
Differential Revision: 484