ci: Drop Node.js v4 testing (as it's not supported for longer while)

Author: medikoo

.travis.yml

@@ -111,9 +111,6 @@ jobs:
         # Upgrade npm to version which is distributed with Node.js v8
         - npm i -g npm@6.4.1
 
-    - name: 'Unit Tests - Node.js v4'
-      node_js: 4
-
     - stage: Tag on Release
       name: 'Tag on Release'
       node_js: 12