adding back apt-get install for npm

.travis.yml

@@ -3,6 +3,7 @@ language: python
 python:
   - "2.7"
 install:
+  - "sudo apt-get install -y npm"
   - "pip install --allow-all-external -r requirements.txt"
   - "pip install --allow-all-external demjson"
   - "sudo npm install -g js-yaml"