Cache ~/.npm instead of node_modules

See https://twitter.com/maybekatz/status/905213355748720640

Author: tanx

.travis.yml

@@ -19,7 +19,7 @@ env:
 
 cache:
   directories:
-  - node_modules
+  - $HOME/.npm
   - $HOME/.cache/electron
   - $HOME/.cache/electron-builder