Remove default encoding from Pygments call

Currently only UTF-8 encoded repositories are supported by Barkeep.

This tiny commit seems to solve all issues with other encodings, for example ISO-8859-1. Apparently, Pygments decides which encoding to use and no exceptions are raised when viewing diffs in non-UTF-8 encoded repositories.

Can anyone confirm that this the way to go? I'm not quite sure if there's a better way to handle those issues. I tried to get the encoding from the actual diff that is passed to Pygments, but that didn't work for me at all.