Remove accidental gap above CodeMirror-gutter elements

Closes #3834

lib/codemirror.css

@@ -197,6 +197,7 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {color: #f22;}
   white-space: normal;
   height: 100%;
   display: inline-block;
+  vertical-align: top;
   margin-bottom: -30px;
   /* Hack to make IE7 behave */
   *zoom:1;