travis; update node versions

Test on active release lines

Author: aheckmann

.travis.yml

@@ -1,12 +1,15 @@
 language: node_js
 node_js:
-  - "4"
-  - "5"
-  - "6"
-  - "7"
   - "8"
-  - "9"
   - "10"
+  - "12"
+matrix:
+  include:
+    - 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"
 script:
   - npm test
   - npm run lint