Consistent histograms from both reservoir implementations

The significant change in this PR is that both reservoir implementations will create equivalent histograms given the same data.  Previously, the implementations could pick different bins for merging when the minimum gaps were tied.  For example, before this PR these may have produced varying histograms:

```clojure
(reduce insert! (create :bins 2 :reservoir :tree) [1 2 3])
(reduce insert! (create :bins 2 :reservoir :array) [1 2 3])
```

The PR also adds some regression tests, some convenience functions, moves to Clojure 1.5.1 and bumps the histogram library to 3.2.0.