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

.travis.yml

@@ -25,13 +25,13 @@ stage: test
 env:
   global:
   - COVERAGE_OPTION='./node_modules/.bin/nyc'
-  - NODE_VERSION=6.11.4
+  - NODE_VERSION=8.11.2
   matrix:
   - MONGODB_VERSION=3.2.13
   - MONGODB_VERSION=3.4.4
   - PARSE_SERVER_TEST_DB=postgres
   - PARSE_SERVER_TEST_CACHE=redis
-  - NODE_VERSION=8.7
+  - NODE_VERSION=stable
 before_install:
 - nvm install $NODE_VERSION
 - nvm use $NODE_VERSION
@@ -50,7 +50,7 @@ jobs:
   include:
     # release on github latest branch
     - stage: release
-      node_js: '6.11.4'
+      node_js: '8.10'
       env: 
       before_script: skip
       after_script: skip