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

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).

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
```

When starting a virtual machine, one can select a platform with the
`PLATFORM` environment variable.  For example:

```
$ PLATFORM=cdh4.5.0 vagrant up
```

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.