Better doc building on travis.

This makes two changes:

* Force `tox` to use a new version of `sphinx`, since we depend on features
  from the `1.3` branch.

* Tweak the `build-docs` script to build on any push. The logic for skipping
  the build in the case of a non-master push was originally in place to
  prevent new docs from being pushed live, but we opted to make that manual
  instead. As a result, this just masks docs errors in pull requests.

scripts/build-docs

@@ -18,13 +18,6 @@
 
 set -e
 
-if [[ "${TRAVIS}" == "true" ]] && \
-     ([[ "${TRAVIS_BRANCH}" != "master" ]] ||
-        [[ "${TRAVIS_PULL_REQUEST}" != "false" ]]); then
-  echo 'Detected non-master push in travis, exiting.'
-  exit 0
-fi
-
 if [[ -z "${SKIP_GAE_SDK}" ]]; then
   scripts/fetch_gae_sdk.py
   export PYTHONPATH="${PWD}/google_appengine:${PYTHONPATH}"

tox.ini

@@ -42,7 +42,7 @@ deps =
     {[testenv:cover]deps}
     python-gflags
     pyyaml
-    sphinx
+    sphinx>=1.3b2
     sphinx-rtd-theme
 commands = {toxinidir}/scripts/build-docs