Archlinux updated OpenSSL to 1.1.x. Earlier versions of ruby (< 2.3)
depend on OpenSSL 1.0. Thus we have to install openssl-1.0 via pacman
and set the environment variables before installing one of the older rubies.
export PKG_CONFIG_PATH=/usr/lib/openssl-1.0/pkgconfig
export CFLAGS="$CFLAGS -I/usr/include/openssl-1.0"
export LDFLAGS="$LDFLAGS -L/usr/lib/openssl-1.0 -lssl"
rvm reinstall ruby-2.1