ci(node): set node 8 to 8.3 and add node 12

Author: mehrad-rafigh

.travis.yml

@@ -1,7 +1,8 @@
 language: node_js
 node_js:
+  - 12
   - 10
-  - 8
+  - 8.3
 
 # Trigger a push build on master and greenkeeper branches + PRs build on every branches
 # Avoid double build on PRs (See https://github.com/travis-ci/travis-ci/issues/1147)