RMagick 3.0.0 has been released on 2019-02-16 but the installation of Redmine fails due to the latest RMagick in some environments.

This is because RMagick 3.0.0 narrowed the supported ImageMagick version. Previous Rmagick 2.16.0 supports ImageMagick >= 6.4.9 but 3.0.0 supports only ImageMagick ImageMagick >= 6.8.9 (even RMagick 3.0.0 does not support ImageMagick 7 yet).

Some Linux distribution still bundles older ImageMagick. For example, CentOS 7 bundles ImageMagick 6.7.8. Rmagick 3.0.0 causes an error during the installation of Redmine in such environments.

There is no benefit for us to update to RMagick 3.0.0 for now, so we should pin the version of RMagick in the Gemfile.

Index: Gemfile
===================================================================
--- Gemfile (revision 17874)
+++ Gemfile (working copy)
@@ -32,7 +32,7 @@
 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

p.s.
RMagick sometimes causes such build errors while installing Redmine. We can escape from such troubles by switching to MiniMagick that is written in pure-Ruby. We already have the patch to remove RMagick (#30492).