Use container-fluid class to prevent inconsistent 'Next' placement

First off thanks for making this! I'm loving it!

Noticed the 'Next' link for each lesson was, depending on screen size, displaying inconsistently or not at all:

![image](https://cloud.githubusercontent.com/assets/1479206/4967976/6876b8fc-682c-11e4-8881-63e425edc501.png) ![image](https://cloud.githubusercontent.com/assets/1479206/4967978/761b37a8-682c-11e4-8176-bc8a407796dc.png)

 ![image](https://cloud.githubusercontent.com/assets/1479206/4967983/829d39d6-682c-11e4-9e50-870e71a8f51d.png)

Using `container-fluid` resolves the issue nicely :smile: