Travis CI: run rstlint.py in the docs job (#68)

Currently, http://buildbot.python.org/all/buildslaves/ware-docs
buildbot is only run as post-commit. For example, bpo-29521 (PR#41)
introduced two warnings, unnotified by the Travis CI docs job.

Modify the docs job to run toosl/rstlint.py.

Fix also the two minor warnings which causes the buildbot slave to
fail.

Author: vstinner

.travis.yml

@@ -40,6 +40,7 @@ matrix:
             make venv PYTHON=python3
       script:
         - make html SPHINXBUILD="./venv/bin/python3 -m sphinx" SPHINXOPTS="-q"
+        - python3 tools/rstlint.py -i tools -i venv
     - os: linux
       language: c
       compiler: clang