Pin sphinx version to 1.7.9

[davidlatwe]

Latest Sphinx release 1.8.0 fail to build doc

Traceback (most recent call last):
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/application.py", line 224, in __init__
    self.setup_extension(extension)
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/application.py", line 449, in setup_extension
    self.registry.load_extension(self, extname)
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/registry.py", line 472, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/builders/applehelp.py", line 20, in <module>
    from sphinx.builders.html import StandaloneHTMLBuilder
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/builders/html.py", line 40, in <module>
    from sphinx.highlighting import PygmentsBridge
  File "/usr/autodesk/maya/lib/python2.7/site-packages/sphinx/highlighting.py", line 26, in <module>
    from sphinx.ext import doctest
SyntaxError: unqualified exec is not allowed in function 'run' it contains a nested function with free variables (doctest.py, line 97)

It has been labeled as bug at Sphinx's issue, and planned to fix it at next release (1.8.1). This PR will pin version to 1.7.9 for the time being.

[mottosso]

Thanks @davidlatwe