enable gzip compression on heroku, but do not compress images

Change Description - :smile:
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### What does this change do?
This PR enables gzip compression on heroku so that the page speed is increased according to Google Page Speed.

![screenshot 2014-05-01 09 30 37](https://cloud.githubusercontent.com/assets/331004/2852443/efbc2322-d134-11e3-8b86-9fe599c3941b.png)

### What testing has been done?
Tests have passed and this has been QA'd locally. Code has been reviewed as well.

Risk Assessment
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### What are the risks? Is there a business impact?
There isn't a risk or a business impact.