(ci): add Travis CI support

- using the default install and default test commands, so that was
  quick
  - use 8.9 to match local environment
    - apparently Node 10 LTS causes the browserify test to error out
      `URL.createObjectURL` in `webworkify` weirdly enough:
      https://travis-ci.org/agilgur5/physijs-webpack/builds/547025740

.travis.yml

@@ -0,0 +1,3 @@
+language: node_js
+# default is apparently 0.10.48 for some reason O.O
+node_js: '8.9'