Update Node testing to current release schedule

Author: jakezatecky

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 node_js:
-  - '8'
   - '10'
   - '12'
+  - '13'
 before_script:
   # Remove ./node_modules/.bin from PATH so node-which doesn't replace Unix which and cause RVM to barf. See https://github.com/travis-ci/travis-ci/issues/5092
   - PATH=${PATH//:\.\/node_modules\/\.bin/}