Test against Node.js v12.0.0-rc.1 on Travis

Author: lukechilds

.travis.yml

@@ -1,5 +1,10 @@
 language: node_js
-node_js: 'stable'
+matrix:
+  include:
+  - node_js: 'stable'
+  - node_js: 'node'
+    env: VERSION=v12.0.0-rc.1
+    before_install: NVM_NODEJS_ORG_MIRROR=https://nodejs.org/download/rc nvm install $VERSION
 script: npm test
 notifications:
   email: