fix(ci): Update Node versions used in tests (optimizely#469)

Summary:

Update TravisCI configuration Node versions: remove 6, add 14.

Test plan:

Existing tests

.travis.yml

@@ -1,10 +1,10 @@
 language: node_js
 node_js:
-- '6'
 - '8'
 - '9'
 - '10'
 - '12'
+- '14'
 branches:
   only:
   - master