Force npm@4.0.0 due to shrinkwrap bug in 3.0.9

https://github.com/npm/npm/issues/14042

.travis.yml

@@ -88,7 +88,11 @@ before_install:
   - "export DISPLAY=:99.0"
   - "/sbin/start-stop-daemon --start -v --pidfile ./tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1920x1080x16"
   - "echo `xdpyinfo -display :99 | grep 'dimensions' | awk '{ print $2 }'`"
+  # Install Node LTS Boron (6.9.1)
   - nvm install 6.9.1
+  # We need npm 4.0 for a bugfix in cross-platform shrinkwrap
+  # https://github.com/npm/npm/issues/14042
+  - npm install npm@4.0
   - travis_retry npm install
 
   # We need phantomjs 2.0 to get tests passing

package.json

@@ -13,6 +13,7 @@
     "typescript": "^2.0.3"
   },
   "engines": {
-    "node": "6.9.1"
+    "node": "6.9.1",
+    "npm": "4.0.0"
   }
 }