[ex/mimemagic/Rakefile] Use older Ruby HEREDOC

The "squiggly" heredoc was introduced in later versions of Ruby, so
using a more compatible version of them with gsubing (to have the same
effect) to allow this to build on more versions of Ruby.

ext/mimemagic/Rakefile

@@ -11,7 +11,7 @@ def locate_mime_database
   path = possible_paths.find { |candidate| File.exist?(candidate) }
 
   return path unless path.nil?
-  raise(<<~ERROR)
+  raise(<<-ERROR.gsub(/^ {3}/, ""))
    Could not find MIME type database in the following locations: #{possible_paths}
 
    Ensure you have either installed the shared-mime-info package for your distribution, or