defaults. The location is dependent on the implementation.
This triggered a build failure on TravisCI because the location
of the default Python binary is different to what is in the Docker
container.
The Linux builds rely on Docker (using Ubuntu 16.04LTS and Ubuntu
14.04LTS) to build and test Z3 so that builds are easily reproducible.
A build status button has been added to `README.md` so that it is
easy to see the current build status.
More documentation can be found in `contrib/ci/README.md`.
This implementation currently tests 13 different configurations. If
build times become too long we can remove some of them.
Although it would be nice to test macOS builds that requires
significantly more work so I have left this as future work.
