Cache the caches to avoid some npm bizarreness when bumping packages.

This changes the Travis caching setup to cache the `.npm` and `.cache`
directories, instead of caching the output of `node_modules` /
`bower_components`. It should still reduce the network time, but allow
the npm installs to work properly when there are package bumps.

Can't make it worse  ¯\_(ツ)_/¯...

.travis.yml

@@ -10,8 +10,8 @@ addons:
 
 cache:
   directories:
-    - node_modules
-    - bower_components
+    - $HOME/.npm
+    - $HOME/.cache # includes bowers cache
 
 before_install:
   - "npm config set spin false"