Only consider target callable if it's a proc

Sequel::Dataset implements the .call method but it means
something different.
http://sequel.jeremyevans.net/rdoc/classes/Sequel/Dataset.html#method-i-call

This conditional is only meant to support delayed scope evaluation,
so checking whether the target is a Proc is more what we're after.

Intended to fix https://github.com/nathanl/searchlight/issues/25