Lock yarn version to fix breakage

Once yarn updates past version 0.23, we can unlock it.

Dockerfile

@@ -14,7 +14,7 @@ RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
 RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
 
 # Install system packages
-RUN apt-get update && apt-get upgrade -y # 23MAR2017
+RUN apt-get update && apt-get upgrade -y # 27APR2017
 RUN apt-get install -y \
      dnsmasq           \
      g++               \
@@ -46,7 +46,7 @@ RUN apt-get install -y \
      python-dev        \
      python-pip        \
      python-virtualenv \
-     yarn              \
+     yarn=0.22.0-1     \
      w3m
 
 # Install python dependencies