Add Travis CI configuration

[jimmysoda]

Create a Travis CI configuration file (.travis.yml) for building the pdal-c master branch.

Refer to the existing GitLab CI configuration file (.gitlab-ci.yml) for an example. If possible, use the Docker images at simverge/pdalc-build-deps for the builds, since they simply extend pdal/pdal Docker images. Dockerfiles for Alpine and Ubuntu images are in the pdal-c repository's tests/docker directory.

Update the root README.md to report the status of the pdal-c build on a badge.