ci: test on node versions 10 and 12

stop testing on versions 6 and 8, as they are now deprecated

Author: dpopp07

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 sudo: false
 node_js:
-- 6
-- 8
+- 10
+- 12
 cache:
   npm: false
 before_install:
@@ -28,5 +28,5 @@ deploy:
   skip_cleanup: true
   script: npx semantic-release
   on:
-    node: 8
+    node: 12
     repo: watson-developer-cloud/node-sdk