via http://qiita.com/yukimura1227/items/740be1aaff186baf7e42

vagrant-setup-rbenv.sh

#!/bin/bash

RBENV_INSTALL_DEST_DIR=/usr/local/rbenv
COMMON_BASH_SETTING_FILE=/etc/profile

echo "install gcc..."
sudo yum -y install gcc

echo "install openssl-devel..."
sudo yum -y install openssl-devel

echo "install git..."
sudo yum -y install git

echo "download rbenv..."
git clone https://github.com/sstephenson/rbenv.git ${RBENV_INSTALL_DEST_DIR}

# add rbenv path To $PATH
echo "append rbenv path info To ${COMMON_BASH_SETTING_FILE}..."
echo "export RBENV_ROOT=\"${RBENV_INSTALL_DEST_DIR}\"" >> ${COMMON_BASH_SETTING_FILE}
echo 'export PATH="${RBENV_ROOT}/bin:$PATH"' >> ${COMMON_BASH_SETTING_FILE}
echo 'eval "$(rbenv init -)"' >> ${COMMON_BASH_SETTING_FILE}

# ${COMMON_BASH_SETTING_FILE} reload
echo "${COMMON_BASH_SETTING_FILE} reload"
source ${COMMON_BASH_SETTING_FILE}

echo "download rbenv plugin(ruby-build)..."
git clone https://github.com/sstephenson/ruby-build.git ${RBENV_INSTALL_DEST_DIR}/plugins/ruby-build

# install ruby
echo "ruby install..."
rbenv install -v 2.1.2

rbenv global 2.1.2

rbenv rehash

# set rbenv owner
chown -R vagrant:vagrant ${RBENV_INSTALL_DEST_DIR}

Vagrantfile

Vagrant::Config.run do |config|
  config.vm.box = "centos64_6_5"
  config.vm.box_url = "http://opscode-vm-bento.s3.amazonaws.com/vagrant/virtualbox/opscode_centos-6.5_chef-provisionerless.box"

  config.vm.provision "shell", path: "setup_rbenv.sh"
end