Merge branch 'ruby-2-3' into 'master'

Update documentation to use Ruby 2.3.1

GitLab is using Ruby 2.3.1 now on master, I figure the documentation should also be updated.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4948

cc: @zj @jacobvosmaer-gitlab 

See merge request !178

.koding.yml

@@ -42,7 +42,7 @@ resource:
         echo _KD_NOTIFY_@Installing RVM and Ruby...@
         # install rvm
         gpg --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
-        curl -sSL https://get.rvm.io | bash -s stable --ruby=2.1.8
+        curl -sSL https://get.rvm.io | bash -s stable --ruby=2.3.1
         source $USER_HOME/.rvm/scripts/rvm
         echo _KD_NOTIFY_@Installing bundler...@

Vagrantfile

@@ -79,8 +79,8 @@ else
 fi
 sudo apt-get install -y rvm \
 	&& sudo addgroup $DEV_USER rvm \
-	&& sudo -u $DEV_USER -i bash -l -c "rvm install 2.1.8 \
-	&& rvm use 2.1.8 --default \
+	&& sudo -u $DEV_USER -i bash -l -c "rvm install 2.3.1 \
+	&& rvm use 2.3.1 --default \
 	&& gem install bundler"
 sudo chown -R $DEV_USER:$DEV_USER /home/vagrant
 sudo ln -s /vagrant /home/vagrant/gitlab-development-kit

doc/prepare.md

@@ -20,7 +20,7 @@ during installation.
 1. A non-root Unix user, this can be your normal user but **DO NOT** run the
    installation as a root user
-2. Ruby 2.1 (2.1.8 or newer) installed with a Ruby version manager (RVM, rbenv,
+2. Ruby 2.3 (2.3.1 or newer) installed with a Ruby version manager (RVM, rbenv,
    chruby, etc.), **DO NOT** use the system Ruby
 3. Bundler, which you can install with `gem install bundler`
@@ -109,8 +109,8 @@ sudo yum install postgresql93-server libicu-devel cmake gcc-c++ redis ed fontcon
 sudo gpg2 --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3
 sudo curl -sSL https://get.rvm.io | bash -s stable
 sudo source /etc/profile.d/rvm.sh
-sudo rvm install 2.1
-sudo rvm use 2.1
+sudo rvm install 2.3
+sudo rvm use 2.3
 #Ensure your user is in rvm group
 sudo usermod -a -G rvm <username>
 #add iptables exceptions, or sudo service stop iptables