Fix the menu icons in IE 11.

According to MDN [1] background-size: 100% 50px; will set the height to be 50px and the width to 100% of the background positioning area. This leads to the image being stretched in IE.

This commit addresses the issue by replacing 100% with auto, so that a proportional width will be used.

This change has been tested in IE 11 and Chrome 31 through the web inspector tools.

[1] - https://developer.mozilla.org/en-US/docs/Web/CSS/background-size?redirectlocale=en-US&redirectslug=CSS%2Fbackground-size