Increase Cassandra startup timeout to 60 seconds

Cassandra boots up on TravisCI within a non-deterministic time (see https://travis-ci.org/spring-projects/spring-data-examples/builds/244420923). Sometimes bootup takes 10 seconds, most of the time 30 seconds. Increasing the timeout from 10 seconds to 60 seconds gives slow CI hosts enough time to start up Cassandra.