Update core:security:bucket() spec

Buckets are binaries, not strings. See basho/riak_kv#920 for more
detail. This was discovered by the way kv was calling
riak_core_security:check_permission/2.
Merge pull request #577 from basho/bugfix/update-bucket-spec

Update core:security:bucket() spec

Reviewed-by: seancribbs