Security "bucket" is a 2-tuple of binary

The security API expects the "bucket" param to be a `{binary(),
binary()}`.  I quote the word bucket because security is actually
broader than buckets but the misnomer remains.

This fixes the following dialyzer error:

```
yz_wm_search.erl:84: The call
riak_core_security:check_permission({_,{_,[any()]}},Security::any())
breaks the contract (Permission::permission(),Context::context()) ->
{'true',context()} | {'false',binary(),context()}
```

This is in reaction to riak_core commit
19be73ab706e3aadbda82047d19f07f0ffc43fe7.