Try to work around CI failure on JRuby 9.4

JRuby rake complains:

"""
You have already activated jar-dependencies 0.4.1, but your Gemfile
requires jar-dependencies 0.5.1.
"""

Since jar-dependencies isn't listed in the Gemfile, this is obviously
false, but maybe it is because jar-dependencies is a dependency of
another gem.  Hopefully specifying the version will fix things.

.ci.gemfile

@@ -61,6 +61,10 @@ platforms :jruby do
   if RUBY_VERSION < '2.5'
     gem 'racc', '< 1.6'
   end
+
+  if RUBY_VERSION >= '3.1'
+    gem 'jar-dependencies', '0.4.1'
+  end
 end
 
 if RUBY_VERSION < '2.2'