Upgrade Commoner and Populist to force upgrade to graceful-fs v2.0.0

A silent upgrade from `graceful-fs` v1.2.2 to v1.2.3 (a dependency for both Commoner and Populist) broke the build process, even though tests were still passing. The 2.0.0 version fixes whatever was broken, though I won't pretend to know exactly what the root cause was.

These upgrades are not strictly necessary if you're doing `npm install` from scratch, but I want to make sure we skip over that bad version of `graceful-fs`.

cc @zpao