Added Ruby gems script

Necessary to provide a single source of truth for installing Ruby
dependencies. This used to be managed by the Ruby Setup project but
that project is being deprecated in favor of this project.

bin/install_ruby_gems

@@ -0,0 +1,17 @@
+#! /usr/bin/env bash
+
+# Installs Ruby gems.
+
+set -o nounset
+set -o errexit
+set -o pipefail
+IFS=$'\n\t'
+
+SCRIPT_PATH="$MAC_OS_CONFIG_PATH/bin/install_ruby_gems"
+
+if [[ -x "$SCRIPT_PATH" ]]; then
+  install_ruby
+  "$SCRIPT_PATH"
+else
+  printf "WARNING: Ruby gems install script does not exist or is not executable.\n"
+fi

bin/run

@@ -39,6 +39,7 @@ while true; do
     printf "     m:  Install Mac App Store software.\n"
     printf "     a:  Install application software.\n"
     printf "     x:  Install application software extensions.\n"
+    printf "    rg:  Install Ruby gems.\n"
     printf "    rc:  Install Rust crates.\n"
     printf "     d:  Apply default settings.\n"
     printf "     s:  Setup installed software.\n"