Fix bug at panels

From http://getbootstrap.com/components/#panels-alternatives
the only panel that the font color is white is
`panel-primary` and for that reason it is the only one that we can't use.

This replace the use of `panel-primary` with `panel-warning`
since prerequisites and learning objectives **never**
appear in the same file.

tools/filters/blockquote2div.py

@@ -52,7 +52,7 @@ SPECIAL_CLASSES = {
     "callout": ("panel-info", "glyphicon-pushpin"),
     "challenge": ("panel-success", "glyphicon-pencil"),
     "prereq": ("panel-warning", "glyphicon-education"),
-    "objectives": ("panel-primary", "glyphicon-certificate"),
+    "objectives": ("panel-warning", "glyphicon-certificate"),
 }