Enable Yarn check files (#3769)

Timer · web-flow · commit 238af4b1dac7 · 2018-01-12T22:14:27.000-05:00
* This is a good default.
Adds approx 4 seconds to install time, but can save some headaches.

* Add no lockfile for add, too
diff --git a/.yarnrc b/.yarnrc
@@ -1 +1,3 @@
 --install.no-lockfile true
+--install.check-files true
+--add.no-lockfile true
