Rubyのバージョンを管理できるrbenvでRubyをシステムワイドに(/usr/local/に)インストールする方法です(Amazon Linuxの場合)。
yum install gcc readline-devel openssl-devel zlib-devel gdbm-devel libffi-devel git make
cd /usr/local
git clone git://github.com/sstephenson/rbenv.git rbenv
mkdir rbenv/{shims,versions}
mkdir /usr/local/rbenv/plugins
cd /usr/local/rbenv/plugins
git clone git://github.com/sstephenson/ruby-build.git
cd ruby-build
./install.sh
echo 'export RBENV_ROOT=/usr/local/rbenv' >> /etc/bashrc
echo 'export PATH="$RBENV_ROOT/bin:$PATH"' >> /etc/bashrc
echo 'eval "$(rbenv init -)"' >> /etc/bashrc
source ~/.bashrc
rbenv install 2.1.2
rbenv global 2.1.2
