[backport to 3.6] bpo-29529: Add .travis.yml to 3.6 branch (#25)

[backport to 3.6] bpo-29529: Add .travis.yml to 3.6 branch (#25)

* Add .travis.yml to 3.6 branch

* 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.

* Only run CI checks when appropriate files have changed (#74)

Closes python/core-workflow#14

* Use 'make check' instead of 'python3 tools/rstlint.py' (#96)