Update README to use pydata instead of pystatsmodels google group

I'm pretty sure PyData is the main mailing list for pandas now, e.g., as seen on the website:
http://pandas.pydata.org/community.html
Merge pull request #8047 from shoyer/patch-1

Update README to use pydata instead of pystatsmodels google group
CI: start using conda
CI: pydata install should use develop