cassandra: updated schema, caching and gc grace

- Realized gc_grace_seconds was on the links table and should have been
  applied to the segments table
- Default cassandra caching is to cache some keys but not values; for
  the links and segments tables we only do long sequential reads so we
  want neither
travis.yml: update to newer cassandra
travis.yml: another cassandra fix
cassandra/helpers: forgotten semicolon deleted
Merge pull request #94 from iParadigms/schema-no-caching

cassandra: updated schema, caching and gc grace