single node version in travis

if multiple node versions set, travis-ci will create a default stage
    called "Test" without env set, and it breaks ember try:one
    $EMBER_TRY_SCENARIO

Author: xg-wang

.travis.yml

@@ -4,8 +4,6 @@ node_js:
   # we recommend testing addons with the same minimum supported node version as Ember CLI
   # so that your addon works for all apps
   - "6"
-  - "8"
-  - "10"
 
 sudo: false
 dist: trusty