build: test against Node.js 13.x nightly

Author: dougwilson

.travis.yml

@@ -17,6 +17,8 @@ matrix:
   include:
     - node_js: "12"
       env: "NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/nightly"
+    - node_js: "13"
+      env: "NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/nightly"
   allow_failures:
     # Allow the nightly installs to fail
     - env: "NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/nightly"