Download and install libicu-dev without using sudo

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.

script/vendor-deb

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+set -ex
+
+cd "$(dirname "$0")/.."
+
+mkdir -p vendor/apt vendor/debs
+
+(cd vendor/apt && apt-get --assume-yes download "$@")
+
+for deb in vendor/apt/*.deb; do
+  ar p $deb data.tar.gz | tar -vzxC vendor/debs --strip-components=2
+done