Bumps minimum node engine to 8+ (#4474)

* Use node 8 as development environment

* fixup! Use node 8 as development environment

* bump node to 8.10

* Targets node 8 for everything

* Run npm install so lock file is up to date

* Use push adapter v3

* Deflake a test on ParseUser

* Adds slight delay after logout

* Ensure we wait even if call fails

* Use node carbon

spec/helper.js

@@ -217,7 +217,14 @@ afterEach(function(done) {
       });
     })
     .then(() => Parse.User.logOut())
-    .then(afterLogOut, afterLogOut)
+    .then(() => {}, () => {}) // swallow errors
+    .then(() => {
+      // Connection close events are not immediate on node 10+... wait a bit
+      return new Promise((resolve) => {
+        setTimeout(resolve, 0);
+      });
+    })
+    .then(afterLogOut)
 });
 
 const TestObject = Parse.Object.extend({