ci(travis): drop tests in node 4 & 9, test in node 10

BREAKING CHANGE: dropped support for node 4

Author: gr2m

.travis.yml

@@ -31,17 +31,13 @@ jobs:
         - npm run start-fixtures-server &
         - sleep 3 # give server some time to start
         - npm run test
-    - node_js: 9
-      env: Node 9, coverage upload
+    - node_js: 10
+      env: Node 10, coverage upload
       script:
         - npm run start-fixtures-server &
         - sleep 3 # give server some time to start
         - npm run test
         - npm run coverage:upload
-    - node_js: 4 # will be removed in v16
-      env: Node 4, smoke test only
-      script:
-        - node index.js
     - node_js: lts/*
       env: browser tests
       script: