Docs fail to build on travis

[manodeep]

General information

• Corrfunc version: 2.3.3
• platform: linux
• installation method (pip/source/other?): source

Issue description

The cron job on Travis failed when trying to build the docs. Turns out this is because sphinx is distributed via pip rather than conda

[manodeep]

Fixed by #215 by a combination of fixes to sphinx installation and installing Corrfunc in editable mode