For Python2.6, downgrade argparse requirement from 1.2.1 to 1.1

Installing nose2 with pip for Python2.6 fails due to the `argparse>=1.2.1` dependency. This is because the argparse entry on PyPI is broken (version 1.2.1 is not actually available!). However, previous versions of argparse can be installed just fine, for example version 1.1.

I just tested changing `argparse>=1.2.1` to `argparse>=1.1` in both `requirements-py26.txt` and `setup.py`. After that, running `tox -e py26` successfully installed into a clean virtualenv and all of the tests passed. Furthermore, after committing and pushing just this change to my fork on github, `pip install git+https://github.com/taleinat/nose2.git#egg=nose2` worked like a charm. 

This fixes the issue which #157 also aims to fix, so choose just one of these fixes.