The minimum version of Pygments on 2.0.x was pretty old, and in practice
most people ended up with a newer version installed anyway. Part of the
problem of having an older version is that modern versions generate HTML
slightly differently, and our unit tests weren't updated for the newer
output.

We now require the same version that Review Board 2.5 requires, and
tests have been updated accordingly.

Installed in a new virtualenv. Unit tests pass.