Add explicit npm i

(Travis might call this automatically before 'script', but we need it
before 'gulp build', and we don't want to do 'gulp build' in 'script',
because the output isn't relevant and if it fails there's no need to run
the tests.)

.travis.yml

@@ -25,6 +25,7 @@ install:
 before_script:
     - export DISPLAY=:99.0
     - sh -e /etc/init.d/xvfb start
+    - npm i
     - node_modules/.bin/gulp build
 script:
     - test/runtests.sh -x firefox/firefox