chore: add nodejs12 to tested version

Author: horike37

.travis.yml

@@ -2,9 +2,10 @@ language: node_js
 
 matrix:
   include:
-    - node_js: '8.9'
-    - node_js: '10.6'
-    - node_js: '10.6'
+    - node_js: 8
+    - node_js: 10
+    - node_is: 12
+    - node_js: 12
       env:
         - DISABLE_TESTS=true
         - LINTING=true