CentOS標準では1.8.7が入ってしまうので、以下の手順を実施しました：
install_ruby.sh
yum update
yum groupinstall "Development Tools"
yum -y install gcc-c++ patch readline readline-devel zlib zlib-devel
yum -y install libyaml-devel libffi-devel openssl-devel make
yum -y install bzip2 autoconf automake libtool bison iconv-devel
gpg2 --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
curl -L get.rvm.io | bash -s stable
rvm install 1.9.3
rvm use 1.9.3 --default