Require sphinx 1.0 to build documentation

Sphinx 0.6.6 in RHEL 6.3 and derivatives is too old, so we
require python-sphinx10 (available form epel) to build docs.
By default docs are not build, so this extra dependency should
not matter much.

Change-Id: I4cc4286917622d50a970e74b2e67c86f5a5643d9