[ruby/digest] Experiment: Use a .pre version in gemspec

This makes it slightly more explicit that this is not a definite new version.

https://github.com/ruby/digest/commit/2bb5bb78a3

ext/digest/digest.gemspec

@@ -3,7 +3,7 @@
 
 Gem::Specification.new do |spec|
   spec.name          = "digest"
-  spec.version       = "3.0.1"
+  spec.version       = "3.0.1.pre"
   spec.authors       = ["Akinori MUSHA"]
   spec.email         = ["knu@idaemons.org"]