We tell apt-get to download it to vendor/apt and then install it into a
vendor/icu directory.
We should be able to just specify --with-icu-dir, but apparently Ruby
2.0 (but not 1.9 or 2.1 or 2.2) has a bug that requires us to use
--with-icu-include/--with-icu-lib instead. Otherwise it can't find the
ICU libraries.
