Drop old docutils versions
A few days ago, docutils-0.11 has been released.
Currently, Sphinx is testing many docutils versions: 0.7, 0.8.1, 0.9.1, 0.10 and 0.11.
Because the docutils is to maintain backward compatibility, it's not complicated to continue support for many older versions. However, test time has been prolonged by this.
I propose that we drop 0.7 and 0.8 versions from tox.ini, but not limit in setup.py.