couchdb: fix libicu linkage

I just toyed around and found that couchdb doesn't start on my machine due to a linking issue.

For some reason, the icu driver library links
with libicu in /usr/local/lib. libicu is keg-only, so this won't
work. CouchDB will fail to start because the driver cannot be loaded.

Solution: patch couch_icu_driver.so using `install_name_tool` so
it links with libicu in the Cellar.

Is there a generic way to do this, my use of `install_name_tool` feels kinda hack-ish.