Stop forcing video player’s speeds widget width

When translating the “Speed” label using a longer word, the speeds
button gets really ugly.  The dropdown and the button will not always
have equal widths, but it’s better than having the text overlap the
other buttons.

common/lib/xmodule/xmodule/css/video/display.scss

@@ -307,9 +307,10 @@ div.video {
             padding-left: 15px;
             position: relative;
             -webkit-font-smoothing: antialiased;
-            width: 116px;
+            min-width: 116px;
 
             @media (max-width: 1024px) {
+              min-width: 0;
               width: 86px;
             }