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 have Ruby 2.1.5 installed. To verify enter ruby -v.
NOTE: I have not tested this so feel free to bring feedback.