Upgrade populist dependency to v0.1.6.

This reduces the time taken by `grunt populist:test` from 7s to 550ms,
which should make @spicyj especially happy.

Relevant commits from the `populist` and `ast-types` repositories:
https://github.com/benjamn/populist/commit/9863ad16c0753f714fdb83007515637511c346d6
https://github.com/benjamn/ast-types/commit/dabef2a4ac48ebdb5c76baf1fb94f4997b6484ac
Merge pull request #672 from benjamn/upgrade-populist

Upgrade populist dependency to v0.1.6.