Add travis and vagrant, use a build matrix to select platform.

See this forum thread:
https://groups.google.com/a/kiji.org/d/topic/dev/_iNQBJa0lCc/discussion

Per the travis user documentation at
* http://docs.travis-ci.com/user/languages/java/
* http://docs.travis-ci.com/user/build-configuration/#The-Build-Matrix

..we enable several parallel builds using the latest cloudera CDH
versions supported by the latest bento (ebi 2.0.1).  The choice of
sub project test suite is also brought into the build matrix to keep
build wall time down (by increasing build parallelism.)

All builds are run with Oracle JDK 7 (travis no longer offers JDK 6
since its End of Life.)

Vagrant enables us to locally reproduce an environment very similar to
travis'.  To select a default provider, echo its name to
`.vagrant/provider`. The default is virtual box.

To always use vmware fusion (for example):
```
$ echo vmware_fusion > .vagrant/provider
```

After installation and configuration, one can create the environment
and run the tests as follows.  Notice the choice of platform with the
`PLATFORM` environment variable.

```bash
$ PLATFORM=cdh4.4.0 vagrant up
$ vagrant ssh
vagrant@kiji:~$ cd kiji
vagrant@kiji:~/kiji$ ./run-tests
vagrant@kiji:~/kiji$ exit
$ vagrant halt  # Or `vagrant destroy -f`
```

The support files for the different platforms themselves are found
under `provisioning/platforms`, each in a titular directory.  Each
such directory contains some common elements:
* a `install` script, which provisions that platform
* a `files` directory containing a file system overlay of
  configuration files and other bits.  Browsing these is the easiest
  way to examine the chosen configuration for each platform.

Generally only the minimal amount of configuration is performed for
each platform.

We also switched to public git urls in `.gitmodules`.  As explained at

http://docs.travis-ci.com/user/build-configuration/#Git-Submodules

..it is better to use public git urls instead of private (ssh-based)
ones.  This allows travis to check the submodules out without an ssh
private key.