Pin icu4c and readline to avoid broken dependency links after Brew upgrades

It's a follow-up after !708 (merged). During the discussion, it was decided to skip pinning the icu4c version in brew because it's necessity was not so obvious (contrary to PKG_CONFIG_PATH customization).

But, I recently came across a case when brew upgrade yarn has caused a broken link between charlock_holmes@0.7.6 (and a couple more gems) and icu4c@63.1 because icu4c was upgraded to 64.2.