Travis: Test eslint 5 with node 6--9

#1843 (review)

Author: papandreou

.travis.yml

@@ -23,6 +23,14 @@ matrix:
   include:
     - node_js: 'lts/*'
       env: PRETEST=true
+    - node_js: '9'
+      env: TEST=true ESLINT=5
+    - node_js: '8'
+      env: TEST=true ESLINT=5
+    - node_js: '7'
+      env: TEST=true ESLINT=5
+    - node_js: '6'
+      env: TEST=true ESLINT=5
     - node_js: '9'
       env: TEST=true ESLINT=next
     - node_js: '8'