Add ability to define plugins' gem dependencies in PluginGemfile (#14030).

git-svn-id: http://svn.redmine.org/redmine/trunk@13337 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -99,7 +99,7 @@ if File.exists?(local_gemfile)
 end
 
 # Load plugins' Gemfiles
-Dir.glob File.expand_path("../plugins/*/Gemfile", __FILE__) do |file|
+Dir.glob File.expand_path("../plugins/*/{Gemfile,PluginGemfile}", __FILE__) do |file|
   puts "Loading #{file} ..." if $DEBUG # `ruby -d` or `bundle -v`
   #TODO: switch to "eval_gemfile file" when bundler >= 1.2.0 will be required (rails 4)
   instance_eval File.read(file), file