Explicitly cache our node_modules in each package

This will massively speed up the install process, which should reduce build times for the next build following this one by ~2/3. Unfortunately, globing isn't allowed, so you'll have to manually add any new packages here. 

Also, the top level node_modules isn't needed, as that's done automatically by Travis. And I changed out the "stable" version for "node", as per Travis's docs.

.travis.yml

@@ -1,14 +1,19 @@
 sudo: false
 language: node_js
 node_js:
-- stable
+  - "node"
 before_install:
   - npm i -g npm
 before_script:
   - npm run build
 cache:
   directories:
-  - node_modules
+  - packages/react-router/node_modules
+  - packages/react-router-dom/node_modules
+  - packages/react-router-native/node_modules
+  - packages/react-router-config/node_modules
+  - packages/react-router-redux/node_modules
+  - packages/react-router-website/node_modules
 branches:
   only:
   - master