Removed system update from Ruby install

Necessary to reduce duplicated effort found in the macOS Configuration
project plus this allows people to customize their Ruby gem install
further in case they don't want this stipulation.

lib/installers.sh

@@ -218,7 +218,6 @@ install_ruby() {
                                             --enable-shared \
                                             --disable-silent-rules
     "$(get_homebrew_bin_root)"/frum local "$version"
-    gem update --system && gem update
   fi
 }
 export -f install_ruby