do not cache node_modules

It's worth taking a bit of extra time to avoid npm cache bugs, and to get a more accurate test of a fresh install on a new user's machine

.travis.yml

@@ -19,6 +19,3 @@ addons:
 before_script:
   - export DISPLAY=':99.0'
   - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
-cache:
-  directories:
-    - node_modules