build: remove Node.js 6 from and add Node.js 13 to travis

Author: Gabriel Schulhof

.travis.yml

@@ -12,10 +12,10 @@ env:
     # https://github.com/jasongin/nvs/blob/master/doc/CI.md
     - NVS_VERSION=1.4.2
   matrix:
-    - NODEJS_VERSION=node/6
     - NODEJS_VERSION=node/8
     - NODEJS_VERSION=node/10
     - NODEJS_VERSION=node/12
+    - NODEJS_VERSION=node/13
     - NODEJS_VERSION=nightly
 matrix:
   fast_finish: true