Adding a bit of padding to invite button

The padding on the invite button looks a bit tight, this loosens it up a bit.

Before:

![screenshot 2014-10-09 14 06 32](https://cloud.githubusercontent.com/assets/65052/4584249/31fe6cac-4ff9-11e4-869b-cbf07f3557c6.png)

After:

![screenshot 2014-10-09 14 06 10](https://cloud.githubusercontent.com/assets/65052/4584255/3ad85ce8-4ff9-11e4-8cd4-0bd8f4921ec9.png)