Stay in RMagick 2.16.0 and don't update to 3.0.0 (#30821).

Patch by Go MAEDA.


git-svn-id: https://svn.redmine.org/redmine/trunk@17875 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -32,7 +32,7 @@ end
 platforms :mri, :mingw, :x64_mingw do
   # Optional gem for exporting the gantt to a PNG file, not supported with jruby
   group :rmagick do
-    gem "rmagick", ">= 2.14.0"
+    gem "rmagick", "~> 2.16.0"
   end
 
   # Optional Markdown support, not for JRuby