Update cassandra driver & stop deleting __columns

The latest version of the cassandra driver hides __columns as per
https://github.com/datastax/nodejs-driver/pull/11#issuecomment-64200962, so we
don't need to explicitly delete __columns any more.

Require the latest cassandra-driver, so that we can rely on this behavior (and
a few other important fixes:
https://datastax-oss.atlassian.net/browse/NODEJS-51?filter=-4&jql=project%20%3D%20NODEJS%20AND%20fixVersion%20%3D%201.0.2).