travis: Use sphinx 1.4 only for now

The sphinx.domains.cpp extension (which I'm not sure we're explicitly using)
generates a warning with parallel docs builds, which causes the Travis build to
fail. This may be something that needs to be fixed upstream.

.travis.yml

@@ -19,7 +19,7 @@ matrix:
             - gcc-4.8-multilib
             - g++-4.8-multilib
 before_install:
-  pip install --user sphinx "requests[security]"
+  pip install --user "sphinx==1.4" "requests[security]"
 script:
 - git tag tmp-travis-build
 - sh travis/git-info.sh