Sets don't seem to handle vector clocks correctly

It's entirely possible I'm overlooking something stupid, but:

````ruby
set = Riak::Crdt::Set.new bucket, "ruby"

set.remove "fritz"
````
"fritz" isn't in the set.

This results in a precondition failure exception, but per @seancribbs Riak will silently accept the removal of non-existent items from a set so long as the vector clock is provided with the removal request.