If you don't want to use Ruby from your repositories and you don't want (or can't because of some reason) to use RVM, here's a quick howto download and compile Ruby from sources (all the commands should be executed with sudo or as a super user):

First a small update and some neccessery libs:

sudo su
apt-get update

# Not sure if all of them are required but nothing bad will happen if you just install them

apt-get install build-essential bison openssl libreadline6 libreadline6-dev \
libyaml-dev libxml2-dev libxslt-dev zlib1g zlib1g-dev libssl-dev autoconf \
libc6-dev ncurses-dev libaprutil1-dev libffi-dev libcurl4-openssl-dev libapr1-dev

Then we should go here and download the most recent stable version:

# Still as root
cd ~
wget http://cache.ruby-lang.org/pub/ruby/2.1/ruby-2.1.2.tar.gz

Of course we need to unpack it:

tar -xvf ruby-2.1.2.tar.gz
# go to where it was unpacked
cd ruby-2.1.2/

and now the whole installation process:

make test  
make install 

and a small cleanup:

cd ~
rm -rf ruby-2.1.2*

To test it, just:

ruby -v
# output: ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-linux]