Changed the width of 'pre' from fixed value to percentage so that it shows correctly in mobile view

I was looking at the website on my phone (Chrome on Nexus 5) and the 'Try it' section CSS was weird. It was something like: 

![exercismmobilebroken](https://cloud.githubusercontent.com/assets/667114/3422581/f5edbf20-ff4d-11e3-85b4-410c6f496ba2.jpg)
Change
---
*The minor fix just changes the fixed width of the element to a relative 50% width. With this change, it will now look something like*:

![exercismmobilefixed](https://cloud.githubusercontent.com/assets/667114/3422580/f5ed5d1e-ff4d-11e3-8428-cb4f5d2416ef.jpg)

I've tested this with local exercism.io setup.

@kytrinyx