Use the yarn pacakge manager for javascript

[Yarn](https://yarnpkg.com/) is a drop in replacement for npm.

We should use it because:

* It's faster
* It uses a lockfile, making the builds more deterministic.

webui/Dockerfile

@@ -1,13 +1,15 @@
-FROM node:6.3.0
+FROM node:6.9.1
 
 ENV WEBUI_DIR /src/webui
 RUN mkdir -p $WEBUI_DIR
 
 COPY package.json $WEBUI_DIR/
+COPY yarn.lock $WEBUI_DIR/
 
 WORKDIR $WEBUI_DIR
 RUN npm set progress=false
-RUN npm install --quiet
+RUN npm install --quiet --global yarn@0.16.1
+RUN yarn install
 
 COPY . $WEBUI_DIR/