travis: Add yarn and nodejs caching (#2504)

Also cache node_module and yarn cache in Travis test process.
Improves speed for Travis tests.

.travis.yml

@@ -1,5 +1,9 @@
 language: ruby
-cache: bundler
+cache:
+  bundler: true
+  yarn: true
+  directories:
+    - node_modules
 dist: trusty
 sudo: false