Make .CodeMirror-selected style less specific

To make it easier for themes to override it.

Fixes ambiance

lib/codemirror.css

@@ -214,8 +214,8 @@ div.CodeMirror span.CodeMirror-nonmatchingbracket {color: #f22;}
   visibility: visible;
 }
 
-div.CodeMirror-selected { background: #d9d9d9; }
-.CodeMirror-focused div.CodeMirror-selected { background: #d7d4f0; }
+.CodeMirror-selected { background: #d9d9d9; }
+.CodeMirror-focused .CodeMirror-selected { background: #d7d4f0; }
 
 .CodeMirror-searching {
   background: #ffa;