Install 'yarn' globally to avoid PATH issues

doc/install/installation.md

@@ -168,8 +168,10 @@ are out of date, so we'll need to install through the following commands:
     curl --location https://deb.nodesource.com/setup_7.x | sudo bash -
     sudo apt-get install -y nodejs
-    # install yarn
-    curl --location https://yarnpkg.com/install.sh | bash -
+    curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
+    echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+    sudo apt-get update
+    sudo apt-get install yarn
 Visit the official websites for [node](https://nodejs.org/en/download/package-manager/) and [yarn](https://yarnpkg.com/en/docs/install/) if you have any trouble with these steps.