build: move node/6 to travis allowed failures and add node/13 (#573)

Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>

Author: Gabriel Schulhof

.travis.yml

@@ -16,11 +16,13 @@ env:
     - NODEJS_VERSION=node/8
     - NODEJS_VERSION=node/10
     - NODEJS_VERSION=node/12
+    - NODEJS_VERSION=node/13
     - NODEJS_VERSION=nightly
 matrix:
   fast_finish: true
   allow_failures:
     - env: NODEJS_VERSION=nightly
+    - env: NODEJS_VERSION=node/6
 sudo: false
 cache:
   directories: