Use brew link instead of manually symlinking ic4c

Simply using `brew link` instead of manually symlinking works as well, and is probably easier to maintain. The `--force` flag is needed because the ic4c library is marked as "keg only". I verified that it worked locally by `unlink`ing the old symlinks, `brew uninstall icu4c` and running these two commands.
Merge pull request #13 from erkie/patch-1

Use brew link instead of manually symlinking ic4c