Fixing gittip badge

The gittip badge from shields.io wasn't displaying properly because I think their SSL Cert has expired or improperly configured on the server:
![Badge screenshot](https://s3.amazonaws.com/f.cl.ly/items/1u2I1r2p261v1x1H2H1Q/Screen%20Shot%202013-12-26%20at%2014.55.39.png)

![Invalid Cert](https://s3.amazonaws.com/f.cl.ly/items/252a0q0B2j3I3S44060D/Screen%20Shot%202013-12-26%20at%2014.56.23.png)

Simply using http solved the problem :smile: