Pin Sphinx to version 2.4.4 on Travis builds (#735)

Sphinx 3 seems to break our docs build.

Author: WasabiFan

.travis.yml

@@ -13,7 +13,7 @@ sudo: false
 git:
   depth: 100
 install:
-- if [ $USE_MICROPYTHON = false ]; then pip install Pillow Sphinx sphinx_bootstrap_theme recommonmark evdev==1.0.0 && pip install -r ./docs/requirements.txt; fi
+- if [ $USE_MICROPYTHON = false ]; then pip install Pillow Sphinx==2.4.4 sphinx_bootstrap_theme recommonmark evdev==1.0.0 && pip install -r ./docs/requirements.txt; fi
 - if [ $USE_MICROPYTHON = true ]; then ./.travis/install-micropython.sh && MICROPYTHON=~/micropython/ports/unix/micropython; fi
 script:
 - chmod -R g+rw ./tests/fake-sys/devices/**/*