correct gitlab version that started requiring yarn

doc/install/installation.md

@@ -155,9 +155,10 @@ page](https://golang.org/dl).
 ## 4. Node
 Since GitLab 8.17, GitLab requires the use of node >= v4.3.0 to compile
-javascript assets and yarn >= v0.17.0 to manage javascript dependencies. In
-many distros the versions provided by the official package repositories are
-out of date, so we'll need to install through the following commands:
+javascript assets, and starting in GitLab 9.0, yarn >= v0.17.0 is required to
+manage javascript dependencies. In many distros the versions provided by the
+official package repositories are out of date, so we'll need to install through
+the following commands:
     # install node v7.x
     curl --location https://deb.nodesource.com/setup_7.x | bash -