3.0-stable: revert r14571 (back to Rails 4.2.3)

git-svn-id: http://svn.redmine.org/redmine/branches/3.0-stable@14574 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -4,7 +4,7 @@ if Gem::Version.new(Bundler::VERSION) < Gem::Version.new('1.5.0')
   abort "Redmine requires Bundler 1.5.0 or higher (you're using #{Bundler::VERSION}).\nPlease update with 'gem update bundler'." 
 end
 
-gem "rails", "4.2.4"
+gem "rails", "4.2.3"
 gem "jquery-rails", "~> 3.1.3"
 gem "coderay", "~> 1.1.0"
 gem "builder", ">= 3.0.4"