Lock npm version on TravisCI

To work around the issue npm/npm#14042,
where npm tries to install optional dependencies and fails.

.travis.yml

@@ -36,6 +36,7 @@ before_install:
 install:
   - nvm install node
   - node -v
+  - npm install -g npm@3.10.7
   - npm config set spin false
   - npm install
   - bundle install