Add test for sdist and pandas import

[ajamian]

As a follow-on to #9827, add a test for packaging pandas and importing that package into a venv that does not contain cython via https://github.com/pydata/pandas/wiki/Release-Checklist:

• python setup.py sdist --formats=zip,gztar
• create virtualenv with pandas dependencies (requirements_dev.txt), minus cython
• pip install <path_to_dist_tar.gz>
• run some subset of nosetests once installed

[jreback]

@ajamian I think we could modify one of the current builds to actually just do this
triggered by a new env variable (this way wouldn't need another new build)

[ajamian]

@jreback sounds good; i was thinking of adding a variable to the 2.6 regression, and add some stuff to the end of script.sh to create the limited venv and then test the install.