Set CASSANDRA_VERSION environment variable for circle tests
Set CASSANDRA_VERSION environment variable for circle tests

This needs to be set for any container that uses a Cassandra
docker-compose file.

EvilMcJerkface/atlasdb,palantir/atlasdb,palantir/atlasdb,EvilMcJerkface/atlasdb,palantir/atlasdb,EvilMcJerkface/atlasdb