the recommended install steps for Node and Yarn are broken

This makes the setup on Circle more explicit, and forces
Yarn to use our version of Node (rather than implicitly
using 9.x) when building native modules, because the
current versions of some of our dependencies  don't have
prebuilt binaries that target these platforms. Maybe
there are updates available. Maybe there's not. Maybe
I don't care and just want this stuff to keep working
like it was a few hours ago.

circle.yml

@@ -18,8 +18,15 @@ dependencies:
 
   pre:
     - brew update
-    - brew upgrade node@8
-    - brew install yarn
+    # uninstall Node 6 which is shipped by default on Circle
+    - brew uninstall node
+    # install latest Node LTS (8.9.3 as of writing)
+    - brew install node@8 --force
+    # everything is terrible, but we must march on
+    - brew link --overwrite node@8 --force
+    # this isn't the recommended way but some dependencies don't
+    # yet have native bindings for Node 9.2.1
+    - npm install -g yarn
 
   override:
     - yarn install --force