Merge branch 'doc/ruby_version_install' into 'master'

Update ruby version for manual installation to 2.1.2

**What does this MR do?**

Updates ruby version from 2.0 to 2.1.2

**Why was this MR needed?**

GitLab dropped support for ruby 1.9.3 and recommends 2.1 therefore docs need update.

**What are the relevant issue numbers?**

#360, #362

/cc @sytse

doc/install/installation.md

@@ -95,8 +95,8 @@ Remove the old Ruby 1.8 if present
 Download Ruby and compile it:
 
     mkdir /tmp/ruby && cd /tmp/ruby
-    curl --progress ftp://ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p481.tar.gz | tar xz
-    cd ruby-2.0.0-p481
+    curl --progress ftp://ftp.ruby-lang.org/pub/ruby/2.1/ruby-2.1.2.tar.gz | tar xz
+    cd ruby-2.1.2
     ./configure --disable-install-rdoc
     make
     sudo make install