Update install.rst

python-social-auth is now available in the Debian repositories. 

# apt-cache policy python-social-auth
python-social-auth:
  Installed: (none)
  Candidate: 0.1.23+gita65b385769-1
  Version table:
     0.1.23+gita65b385769-1 0
        500 http://http.debian.net/debian/ jessie/main amd64 Packages

Note, I wasn't able to test this with Ubuntu, so this still needs to be updated with current Ubuntu information.