[ruby/bigdecimal] Version 3.1.1

https://github.com/ruby/bigdecimal/commit/2ef67c2cc7

ext/bigdecimal/bigdecimal.gemspec

@@ -2,7 +2,7 @@
 
 Gem::Specification.new do |s|
   s.name          = "bigdecimal"
-  s.version       = "3.1.0"
+  s.version       = "3.1.1"
   s.authors       = ["Kenta Murata", "Zachary Scott", "Shigeo Kobayashi"]
   s.email         = ["mrkn@mrkn.jp"]