Update docs to reference Ruby 2.3.1

doc/install/installation.md

@@ -108,8 +108,7 @@ Then select 'Internet Site' and press enter to confirm the hostname.
 
 ## 2. Ruby
 
-_**Note:** The current supported Ruby version is 2.1.x. Ruby 2.2 and 2.3 are
-currently not supported._
+_**Note:** The current supported Ruby version is 2.3.x.
 
 The use of Ruby version managers such as [RVM], [rbenv] or [chruby] with GitLab
 in production, frequently leads to hard to diagnose problems. For example,
@@ -124,9 +123,9 @@ Remove the old Ruby 1.8 if present:
 Download Ruby and compile it:
 
     mkdir /tmp/ruby && cd /tmp/ruby
-    curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.8.tar.gz
-    echo 'c7e50159357afd87b13dc5eaf4ac486a70011149  ruby-2.1.8.tar.gz' | shasum -c - && tar xzf ruby-2.1.8.tar.gz
-    cd ruby-2.1.8
+    curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.1.tar.gz
+    echo 'c39b4001f7acb4e334cb60a0f4df72d434bef711  ruby-2.3.1.tar.gz' | shasum -c - && tar xzf ruby-2.3.1.tar.gz
+    cd ruby-2.3.1
     ./configure --disable-install-rdoc
     make
     sudo make install