Add basic travis-ci support

This includes a basic .travis.yml file for testing against the following JVM
variants:

- openjdk7
- oracle7
- openjdk6

Although we are looking at dropping 6 for future projects, PuppetDB still uses
this so its worth covering.

It also adds the test script to ext/travisci/test.sh which for now just runs
'lein2 test'.

Success for these tests can be seen here: https://travis-ci.org/kbarber/clj-kitchensink/builds/14196480

Signed-off-by: Ken Barber <ken@bob.sh>