Add yarn as npm client package manager replacement

Author: edwinvdgraaf

Dockerfile

@@ -43,6 +43,8 @@ RUN \
   rm -rf /tmp/node-v* && \
   echo -e '\n# Node.js\nexport PATH="node_modules/.bin:$PATH"' >> /root/.bashrc
 
+RUN npm install yarn@0.16.1 -g
+
 # Download phantomjs build (see README)
 RUN \
   cd /tmp && \

spec/dockerfile_spec.rb

@@ -53,6 +53,10 @@ def remove(*); end
     its(:stdout) { should match /3\.10\.3/ }
   end
 
+  describe command("yarn --version") do
+    its(:stdout) { should match /0\.16\.1/ }
+  end
+
   describe command("phantomjs -v") do
     its(:stdout) { should match /2\.0\.0/ }
   end