remove ruby upgrade guide

doc/update/README.md

 - [The individual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update)
 - [Upgrader](upgrader.md)
-- [Ruby](ruby.md)
 - [Patch versions](patch_versions.md)
 - [MySQL to PostgreSQL](mysql_to_postgresql.md)

doc/update/ruby.md

-# Updating Ruby from source
-
-This guide explains how to update Ruby in case you installed it from source according to the [instructions](../install/installation.md#2-ruby).
-
-## 1. Look for Ruby versions
-
-This guide will only update `/usr/local/bin/ruby`. You can see which Ruby binaries are installed on your system by running:
-
-```bash
-ls -l $(which -a ruby)
-```
-
-## 2. Stop GitLab
-
-```bash
-sudo service gitlab stop
-```
-
-## 3. Install or update dependencies
-
-Here we are assuming you are using Debian/Ubuntu.
-
-```bash
-sudo apt-get install build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libreadline-dev libncurses5-dev libffi-dev curl
-```
-
-## 4. Download, compile and install Ruby
-
-Find the latest stable version of Ruby 1.9 or 2.0 at <https://www.ruby-lang.org/en/downloads/>. We recommend at least 2.0.0-p353, which is patched against [CVE-2013-4164](https://www.ruby-lang.org/en/news/2013/11/22/heap-overflow-in-floating-point-parsing-cve-2013-4164/).
-
-```bash
-cd /tmp
-curl --progress http://cache.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p353.tar.gz | tar xz
-cd ruby-2.0.0-p353
-./configure --disable-install-rdoc
-make
-sudo make install # overwrite the existing Ruby in /usr/local/bin
-sudo gem install bundler
-```
-
-### 5. Reinstall GitLab gem bundle
-
-Just to be sure we will reinstall the gems used by GitLab. Note that the `bundle install` command [depends on your choice of database](../install/installation.md#install-gems).
-
-```bash
-cd /home/git/gitlab
-sudo -u git -H rm -rf vendor/bundle  # remove existing Gem bundle
-sudo -u git -H bundle install --deployment --without development test mysql aws # Assuming PostgreSQL
-```
-
-## 6. Start GitLab
-
-We are now ready to restart GitLab.
-
-```bash
-sudo service gitlab start
-```
-
-## Done