Attempt to cache the sphinx-build intermediates on Travis.

.travis.yml

@@ -11,6 +11,7 @@ cache:
   # See http://docs.travis-ci.com/user/caching/#pip-cache
   directories:
     - $HOME/.cache/pip
+    - $HOME/.cache/sphinx
     - $HOME/.ccache
 addons:
   apt:

tools/travis_script.sh

@@ -9,7 +9,7 @@ section_end "Test.with.min.requirements"
 
 section "Build.docs"
 if [[ ($PY != 2.6) && ($PY != 3.2) ]]; then
-    make html
+    SPHINXBUILD="python $(which sphinx-build) -d $HOME/.cache/sphinx" make html
 fi
 section_end "Build.docs"