Add support for dsc21

Features:

- Bumped up foodcritic to '~> 4.0'
- Updated `serverspec` to the `2.0` series, and updated the tests to reflect that.
- Creates dir `/usr/share/java` if it doesn't exist
- New attribute `node[:cassandra][:setup_jamm]` (default: false): install the jamm jar file. The rpm doesn't seem to create this jar file by default.
- Added new `test-kitchen` test suite `dsc21` to start playing around with the new version of cassandra. Will add more tests in future PRs
- Fix: `default[:cassandra][:version] = '2.0.9'` was missing from `datastax.rb` attribute file.

Fixes: https://github.com/michaelklishin/cassandra-chef-cookbook/issues/122