Adds 7.10.1 version (#12)

Signed-off-by: jmnsf <jmnsferreira@gmail.com>

Dockerfile

@@ -6,7 +6,7 @@ MAINTAINER Stefano Sala <stefano@conversio.com>
 # extras and a different node version.
 
 # Which version of node?
-ENV NODE_ENGINE 8.1.3
+ENV NODE_ENGINE 7.10.1
 
 # Locate our binaries
 ENV PATH /app/heroku/node/bin/:/app/user/node_modules/.bin:$PATH
@@ -21,5 +21,10 @@ RUN curl -s https://s3pository.heroku.com/node/v$NODE_ENGINE/node-v$NODE_ENGINE-
 # Export the node path in .profile.d
 RUN echo "export PATH=\"/app/heroku/node/bin:/app/user/node_modules/.bin:\$PATH\"" > /app/.profile.d/nodejs.sh
 
+# Install yarn
+RUN curl -sS http://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
+RUN echo "deb http://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
+RUN apt-get update && apt-get install -y yarn
+
 # Replace shell with bash so we can source files
 RUN rm /bin/sh && ln -s /bin/bash /bin/sh