Fix Ruby 2.7 deprecation warning for Proc.new

Capturing the block implicitly using this mechanism is going to be
removed in Ruby 3.0 [1], so we need to explicitly pass the block in
future.

This change is backwards-compatible with other Ruby versions, it's just
slightly slower (~10x) [2].

[1]: https://bugs.ruby-lang.org/issues/10499
[2]: https://medium.com/@amliving/proc-new-trick-c1df16185599

lib/statesman/adapters/active_record.rb

@@ -197,8 +197,8 @@ def with_updated_timestamp(params)
     end
 
     class ActiveRecordAfterCommitWrap
-      def initialize
-        @callback = Proc.new
+      def initialize(&block)
+        @callback = block
         @connection = ::ActiveRecord::Base.connection
       end