Bump geojson-vt dep version

[kristfal]

Hey,

upstream fix for #1507 was fixed a while back, but hasn't been included in gl-js. Did a test build with 2.1.6 of geojson-vt and can verify that #1507 was resolved.

[mourner]

"^2.1.0" should install "2.1.6", so the next build should include the latest version. However we need to make sure to run npm update before publishing a version. cc @jfirebaugh @bhousel

[jfirebaugh]

We do automated release builds via CI and geojson-vt 2.1.6 should have been pulled into mapbox-gl 0.11.1. The problem is that CI has caching for node_modules, and even a new branch will pull the previous cache from master. So we need to switch the install step in CI to use npm update instead of npm install.

[lucaswoj]

Looks like this has cased some bugs in v0.11.2 🙀 Going to try to fix the deploy script and publish a new version tonight.

[lucaswoj]

Fixed as of #1683