Media query so text fits in call-to-action button

Fix is for this bug report: https://assembly.com/runbook/bounties/160

Added a media query to reduce the text/side padding on the call-to-action button on smaller (mobile) screens. Removed !important flags, as the specificity seems to be high enough that they aren't needed.

Previous screenshot:
![screen shot 2014-12-18 at 9 35 22 am](https://cloud.githubusercontent.com/assets/1382445/5492768/3810ae1c-8699-11e4-85ed-3d58abd87e2e.png)

After the media query is added:
![screen shot 2014-12-18 at 9 34 36 am](https://cloud.githubusercontent.com/assets/1382445/5492755/1fe74b20-8699-11e4-9eaa-ed05083d9f48.png)