Fix fork me github ribbon (#130)

After a wild search (thanks to my google-fu) and extensive testing on rtfd.org, here is a "dirty" fix. It turns out rtfd.org is overriding the `layout.html` as you can read in https://github.com/rtfd/readthedocs.org/issues/152. You can see the results of my testing branch [here](http://fedmsg2.readthedocs.org/en/latest/index.html). I also changed the url of the png to be more human readable.

PS. In order to build the docs on rtfd.org I had to add in requirements: `nose`, `cloud_sptheme` and `fedmsg_meta_fedora_infrastructure`. On my machine (using virtualenv), I had to install those three and also `twitter`, `twitterapi` otherwise it would stop building with an error. This isn't documented anywhere, I wonder if this is just me.