Make Max and Min aggs use <=> instead of >/<

This comparator is more general, and accepts types like Arrays. This is
really useful for vectors, so we can now take the max of vectors like
[1, 2, 3] and [1, 2, 4].

Also added the same duplication case from Min for Max as well. I'm not
certain why they were different, but I can't think of a reason why
max should not return all maximum results, rather than an arbitrary one.

This change is motivated by our CS194-17 project and, uh, our solution
won't run if this isn't patched. So we hope you find the change good!
Merge pull request #308 from ilikerps/master

Make Max and Min aggs use <=> instead of > or <

Apparently Ruby defines Array#<=> but doesn't have Array include Comparable, so this expands what you can apply the min/max aggs to.