Release Memento version 0.1.5.
Release Memento version 0.1.5.

 * Remove Riffle dependency.
 * Upgrade `b-tree` to `2.0.0-alpha.96`.
 * Upgrade `amalgamate` to `0.1.54`.
 * Upgrade `riffle` to `2.0.0-alpha.70`.

Closes #97.

bigeasy/memento