Fix hover on textbook buttons

lms/static/sass/_textbook.scss

@@ -62,6 +62,8 @@ div.book-wrapper {
         @extend .clearfix;
 
         li {
+          background-color: darken($cream, 4%);
+
           &.last {
             display: block;
             float: left;
@@ -77,6 +79,10 @@ div.book-wrapper {
             display: block;
             float: right;
           }
+
+          &:hover {
+            background: none;
+          }
         }
       }