requirements.txt should have sphinx-build

[bartvde]

since it's needed for paver install

bartvde@bartvde-XPS-13-9360:~/opengeo/git/qgis-geogiglight-plugin$ paver install
---> pavement.install
---> pavement.builddocs
git submodule init
git submodule update
make html
sphinx-build -b html -d build/doctrees   source build/html
make: sphinx-build: Command not found
Makefile:44: recipe for target 'html' failed
make: *** [html] Error 127


Captured Task Output:
---------------------

---> pavement.install
---> pavement.builddocs
git submodule init
git submodule update
make html

Build failed running pavement.install: Subprocess return code: 2