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.