Add Node 12 and remove Node 4/6 from Travis

Author: yannickcr

.travis.yml

@@ -1,11 +1,10 @@
 sudo: false
 language: node_js
 node_js:
+  - '12'
   - '11'
   - '10'
   - '8'
-  - '6'
-  - '4'
 before_install:
   - 'nvm install-latest-npm'
 before_script:
@@ -27,8 +26,5 @@ matrix:
   include:
     - node_js: 'lts/*'
       env: PRETEST=true
-  exclude:
-    - node_js: '4'
-      env: ESLINT=5
   allow_failures:
     - node_js: '11'