Cleanup gemspec

I've cleaned up gemspec mess and also removed dependency on `avl_tree`. Correct me if I'm wrong, but it looks like it's used only in `benchmark/samplers.rb` which shouldn't be included in gem bundle either way.
`avl_tree` is used here: https://github.com/eric/metriks/blob/b3ba1a8bd3915835c979cd8f231c4d6333056f0a/lib/metriks/exponentially_decaying_sample.rb#L2