Update Vagrantfile for latest Cassandra versions
Don't reset model updater/deleter if save results in an error

Since update operations are wrapped in a batch, actual error does not occur in
the code path of `#update`. This can result in a record being in a state where
it thinks it was successfully saved, but was not. Instead, we add the ability
to hook into a batch's completion, and then use that to only reset the model's
updaters when it is actually written to the database
Merge pull request #157 from outoftime/dont-clear-dirty-on-error

Don't reset model updater & deleter if save results in an error
Allow setting a key attribute to what it already is

Generally, attempting to set the key attribute on a persisted record is a
no-go. However, if you're setting it to the same thing it already is, no harm
done; it's just a no-op.
Merge pull request #158 from outoftime/allow-noop-key-update

Allow setting a key attribute to what it already is
New Cassandra versions
Bump version
Use WWTD to run tests

Much smoother experience than manually switching Rubies, particularly on the
installation front.

Keep Appraisal around to generate the Gemfiles, which is convenient.