MDL-38850 Horizontal scroll bar added in Lesson

Earlier, horizontal scroll bar is not present in a Lesson page where the content on that page is very wide.

Fixed

Snapshot before Patch:
![before_patch](https://f.cloud.github.com/assets/939075/411943/53416d6a-ab9a-11e2-8594-a36821c06000.png)

Snapshot after Patch:
![after_patch](https://f.cloud.github.com/assets/939075/411944/5c6e3044-ab9a-11e2-9fea-893260616058.png)