byte-size fix for 2 lines page navs

lms/static/sass/course/layout/_courseware_header.scss

@@ -18,7 +18,7 @@
     padding: ($baseline*0.75) 0 ($baseline*0.75) 0;
 
     li {
-      @include float(left);
+      display: inline-block;
       list-style: none;
 
       &.prominent {
@@ -41,12 +41,13 @@
         display: block;
         text-align: center;
         text-decoration: none;
+        border-bottom: 4px solid transparent;
 
         &:hover,
         &:focus,
         &.active {
           color: $uxpl-blue-hover-active;
-          border-bottom: 4px solid $uxpl-blue-hover-active;
+          border-bottom-color: $uxpl-blue-hover-active;
         }
       }
     }