Drop node 4, add node 8 for CI (microsoft#19617)

Author: weswigham

.travis.yml

@@ -2,8 +2,8 @@ language: node_js
 
 node_js:
   - 'stable'
+  - '8'
   - '6'
-  - '4'
 
 sudo: false
 

netci.groovy

@@ -5,7 +5,7 @@ import jobs.generation.Utilities;
 def project = GithubProject
 def branch = GithubBranchName
 
-def nodeVersions = ['stable', '6', '4']
+def nodeVersions = ['stable', '8', '6']
 
 nodeVersions.each { nodeVer ->