Update Rails to 5.2.1.

git-svn-id: https://svn.redmine.org/redmine/trunk@17464 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", "5.2.0"
+gem "rails", "5.2.1"
 gem "coderay", "~> 1.1.1"
 gem "request_store", "1.0.5"
 gem "mime-types", "~> 3.0"