Add runtime tests to Travis CI, in addition to our existing build tests

[bimimicah]

We have tests to make sure code changes build properly on Mac, Windows, and Linux.

It would be helpful to add tests for the actual functionality. Example:

• [x] add mod_authnz_external to apache
• [ ] set up an example authenticator/authorizer
• [ ] set example configuration directives in apache conf files
• [ ] run curl on it several times