Install sphinx_rtd_theme when running sphinx tests.

.travis.yml

@@ -21,7 +21,7 @@ install:
     - if [[ $TRAVIS_PYTHON_VERSION == '3.2' && $DEPS == true ]]; then pip install mock; fi
     # Twisted runs on 2.x and 3.3+, but is flaky on pypy.
     - if [[ $TRAVIS_PYTHON_VERSION != '3.2' && $TRAVIS_PYTHON_VERSION != 'pypy' && $DEPS == true ]]; then pip install Twisted; fi
-    - if [[ $TRAVIS_PYTHON_VERSION == '3.4' && $DEPS == true ]]; then pip install sphinx==1.2.2; fi
+    - if [[ $TRAVIS_PYTHON_VERSION == '3.4' && $DEPS == true ]]; then pip install sphinx==1.2.2 sphinx_rtd_theme; fi
     # On travis the extension should always be built
     - if [[ $TRAVIS_PYTHON_VERSION != 'pypy' ]]; then export TORNADO_EXTENSION=1; fi
     - python setup.py install

tox.ini

@@ -285,6 +285,8 @@ setenv = LANG=en_US.utf-8
 [testenv:py-docs]
 basepython = python3
 changedir = docs
-deps = sphinx==1.2.2
+deps =
+     sphinx==1.2.2
+     sphinx_rtd_theme
 commands =
     sphinx-build -q -E -n -W -b html . {envtmpdir}/html