change exp bar style

Suggestion from a student.
http://coursemology.org/courses/97/forums/suggestions-on-how-to-improve-coursemology--2/topics/exp-bar

 Now exp bar looks like this:

![screen shot 2014-09-01 at 9 10 58 pm](https://cloud.githubusercontent.com/assets/4983239/4108627/0baab1d8-31da-11e4-86af-5c474432e7b3.png)

Please follow houndci's suggestion.