ci(travis): remove tests in Node 6

BREAKING CHANGE: drop support for Node 6

Author: gr2m

.travis.yml

@@ -33,8 +33,7 @@ jobs:
         - 'git commit -a -m "build: known webhooks" --author="Octokit Bot <[email protected]>" && git push "https://${GH_TOKEN}@github.com/$TRAVIS_REPO_SLUG" ${TRAVIS_BRANCH} || true'
 
     - stage: test
-      node_js: 6
-    - node_js: 8
+      node_js: 8
     - node_js: 10
 
     # run semantic-release and coverage in parallel