Most projects now use sphinx 1.5.
sphinx 1.5 provides some new directives which are possibly useful.
sphinx 1.5 is a useful feature which considers warnings as error.
It prevents sphinx warnings into documents.
Also fixes existing sphinx warning.
At the same time, requirements.txt is synced with global-requirements.
Change-Id: I3b4a18a2483fa6966d8d4ac78dcb8ceac0524cf7