Allow testing against Ruby 1.8.7

by only using RuboCop with Ruby 1.9.2 or higher
Let Travis test against 1.8.7 too
Merge pull request #18 from Jimdo/test_ruby_1_8

Allow testing against Ruby 1.8.7
Only return ElastiCache cache cluster nodes that are in state "available"

Affects the behavior of:

 - `redis_cluster_nodes_for_cfn_stack`
 - `memcached_cluster_nodes_for_cfn_stack`
Only return Redis replication groups that are in state "available"

Affects the behavior of:

 - `redis_cluster_replica_groups_for_cfn_stack`