fix yarn installation

scripts/install-node

@@ -2,7 +2,10 @@
 set -xeo pipefail
-curl --silent --location https://deb.nodesource.com/setup_7.x | bash -
-apt-get install -y nodejs
-curl -o- -L https://yarnpkg.com/install.sh | bash
+# add official debian repos for node and yarn
+curl -sS -L https://deb.nodesource.com/setup_7.x | bash -
+curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
+echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
+apt-get update
+apt-get install -y nodejs yarn