Now that we're on the Travis CI container based infrastructure we can
use caching for the local Maven repository [1] in order to speed up the
build times and reduce the load on Maven Central.

 [1] https://docs.travis-ci.com/user/caching/

Sine we're not using sudo we can migrate to the container-based Travis
build infrastructure [1]. This has the following advantages:

 - Builds start in seconds
 - More available resources
 - Better network capacity, availability and throughput
 - Caching available for open source projects

The builds will still fail due to an `OutOfMemoryError` but that's a
different issue.

 [1] https://docs.travis-ci.com/user/migrating-from-legacy/

Run the tests on Travis so that when a pull request breaks some tests
we notice it immediately.

To verify compatibility and for potential JSR-310 support
it would be good if we test against Java 8 as well.

since we no longer support jdk6

Added Travis CI config