build: drop support for Node versions 6 and 8 (#33)

* build: drop support for Node versions 6 and 8

* ci: stop running builds on node 6 and 8

BREAKING CHANGE: This SDK may no longer work with applications running on Node 6 or 8.

Author: dpopp07

migration-guide.md

@@ -0,0 +1,4 @@
+# Migration Guide for v1
+
+## Breaking Changes
+Node versions 6 and 8 are no longer supported.

package.json

@@ -69,7 +69,7 @@
     "vcap_services": "~0.3.4"
   },
   "engines": {
-    "node": ">=6"
+    "node": ">=10"
   },
   "scripts": {
     "commitmsg": "commitlint -E GIT_PARAMS",