Fixing bug with fetch_gist returning bytes over string

Hi,

pelican-gist was breaking for me with python 3.3, and I figured out it has to do with requests returning a 'bytes' object over the expected string object for fetch_gist. It looks like it was desired for fetch_gist to return a string with the contents, so here's a fix for that.

Tested with Python 3.3 and 2.7.

Thanks!