Add missing require for zlib

Zlib is used to generate the advisory lock since commit 2c2a875546 .

Using the Migrator fails since then, when it is called without the rails context:

    NameError: uninitialized constant ActiveRecord::Migrator::Zlib

This patch fixes the above error.

activerecord/lib/active_record/migration.rb

@@ -1,4 +1,5 @@
 require "set"
+require "zlib"
 require "active_support/core_ext/module/attribute_accessors"
 require "active_support/core_ext/regexp"