Use git version tag like "v1.11.0" instead of "1.11.0"

`version_tag` is defined with prefix "v" here:
  https://github.com/ruby/rubygems/blob/569c5c7450137839ff7c536f160b06a43ea3dfe4/bundler/lib/bundler/gem_helper.rb#L193

Author: larskanis

build/gem_helper.rb

@@ -24,10 +24,6 @@ def reldate
       Time.now.strftime("%Y-%m-%d")
     end
 
-    def version_tag
-      "#{version}"
-    end
-
     def update_history
       hin = File.read(hfile)
       hout = hin.sub(/#{headline}/) do