ci: update node 8 -> 12

Author: nodkz

.travis.yml

@@ -8,7 +8,7 @@ notifications:
   email: true
 node_js:
   - "10"
-  - "8"
+  - "12"
 before_install: yarn global add greenkeeper-lockfile@1
 before_script: greenkeeper-lockfile-update
 after_script: greenkeeper-lockfile-upload