update ruby setup instructions

Author: lrytz

README.md

@@ -4,11 +4,15 @@
 
 This repository contains the source for the Scala documentation website, as well as the source for "Scala Improvement Process" (SIP) documents.
 
+## Dependencies ##
+
+This site uses a Jekyll, a Ruby framework. You'll need Ruby and Bundler installed; see [Jekyll installation instructions](https://jekyllrb.com/docs/installation/) for the details.
+
 ## Quickstart ##
 
 To build and view the site locally:
 
-    gem install --user-install bundler jekyll
+    bundle install
     bundle exec jekyll serve -I
 
 ([Trouble on MacOS?](https://github.com/scala/docs.scala-lang/issues/1150))
@@ -53,10 +57,6 @@ Small changes, or corrected typos will generally be pulled in right away. Large
 existing documents will be thoroughly reviewed-- please keep in mind that, generally, new documents must be very well-polished, complete, and maintained
 in order to be accepted.
 
-## Dependencies ##
-
-This site uses a Jekyll, a Ruby framework. You'll need Ruby and Bundler installed; see [Jekyll installation instructions](https://jekyllrb.com/docs/installation/) for the details.
-
 ## Building & Viewing ##
 
 cd into the directory where you cloned this repository, then install the required gems with `bundle install`. This will automatically put the gems into `./vendor/bundle`.