Following are the commands to install Ruby 2.1 in a Debian based system according the the official Ruby Dockerfile. BTW, you can avoid this pain using Docker.
# Update your system packages
apt-get update
# Install some basic dependencies
# Few of thems aren't needed but won't hurt anyway
apt-get install -y \
autoconf \
build-essential \
imagemagick \
libbz2-dev \
libcurl4-openssl-dev \
libevent-dev \
libffi-dev \
libglib2.0-dev \
libjpeg-dev \
libmagickcore-dev \
libmagickwand-dev \
libmysqlclient-dev \
libncurses-dev \
libpq-dev \
libreadline-dev \
libsqlite3-dev \
libssl-dev \
libxml2-dev \
libxslt-dev \
libyaml-dev \
zlib1g-dev \
apt-get install -y curl procps
RUBY_MAJOR="2.1"
RUBY_VERSION="2.1.5"
apt-get install -y bison ruby
mkdir -p /usr/src/ruby
curl -SL "http://cache.ruby-lang.org/pub/ruby/$RUBY_MAJOR/ruby-$RUBY_VERSION.tar.bz2" | tar -xjC /usr/src/ruby --strip-components=1
cd /usr/src/ruby
autoconf
./configure --disable-install-doc
make -j"$(nproc)"
apt-get purge -y --auto-remove bison ruby
make install
rm -r /usr/src/ruby
GEM_HOME="/usr/local/bundle"
PATH=$PATH:$GEM_HOME/bin
gem install bundler
bundle config --global path "$GEM_HOME"
bundle config --global bin "$GEM_HOME/bin"
As you can imagine, some of the commands will need to be enter in sudo mode.
At this point you should be able to use the irb enteringhave Ruby 2.1.5 installed. To verify enter irbruby -v in a terminal.
NOTE: I have not tested this so feel free to bring feedback.