Merge pull request #39 from netlify/yarn_per_node_version

Install Yarn with each selected version.

Dockerfile

@@ -122,15 +122,15 @@ USER buildbot
 RUN git clone https://github.com/creationix/nvm.git ~/.nvm
 
 ENV ELM_VERSION=0.17.1
+ENV YARN_VERSION=0.18.1
 
 RUN /bin/bash -c ". ~/.nvm/nvm.sh && \
 		  nvm install 4 && nvm use 4 && npm install -g sm grunt-cli bower elm@$ELM_VERSION && \
+      		  bash /usr/local/bin/yarn-installer.sh --version $YARN_VERSION && \
 		  nvm install 6 && nvm use 6 && npm install -g sm grunt-cli bower elm@$ELM_VERSION && \
+      		  bash /usr/local/bin/yarn-installer.sh --version $YARN_VERSION && \
 		  nvm alias default node && nvm cache clear"
 
-ENV YARN_VERSION=0.18.1
-RUN bash /usr/local/bin/yarn-installer.sh --version $YARN_VERSION
-
 USER root
 
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