Initial commit of .travis.yml

This should trigger a Travis CI build.  More changes are forthcoming based on the results of that.
Use sudo when installing apt packages
Second try on fixing apt-get command
Add apt-get update to .travis.yml
Create local_settings.py.travis
Copy local_settings.py and create DB user in .travis.yml
Add missing MIDDLEWARE_LOCAL setting
Create database for testing in .travis.yml
Merge branch 'main' into travis-ci-support
Add Selenium test command
Remove selenium tests from .travis.yml

It may be possible to get Selenium tests working in the future, but it's a nontrivial task as they are not directly supported by Travis.  I would like our Travis builds to complete successfully once we've got the kinks out of the main test suite.  It may be a while before the Selenium tests are working.
Merge pull request #763 from learning-unlimited/travis-ci-support

Travis CI support