boost: revision for icu4c

The default configuration of `boost` does not depend on `icu4c`, but the
somewhat popular formula `ncmpcpp` depends on `boost --with-icu4c`. Bump
revision as there's no other way to trigger a rebuild of `boost` for
those users (and other users who have installed it with `--with-icu4c`).
This is necessary for `boost` to pick up the updated `icu4c` and thus
fix the dylib linkage for the affected users.

Fixes #395.

Closes #441.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Formula/boost.rb

@@ -3,7 +3,7 @@ class Boost < Formula
   homepage "https://www.boost.org/"
   url "https://downloads.sourceforge.net/project/boost/boost/1.60.0/boost_1_60_0.tar.bz2"
   sha256 "686affff989ac2488f79a97b9479efb9f2abae035b5ed4d8226de6857933fd3b"
-  revision 1
+  revision 2
 
   head "https://github.com/boostorg/boost.git"