Add ENV['RACK_ENV'] support to rake runner/console/server.

[kennyj]

rake dbconsole support ENV['RACK_ENV']. And railties/lib/rails.rb also support one.

 87     def env
 88       @_env ||= begin
 89         ENV["RAILS_ENV"] ||= ENV["RACK_ENV"] || "development"
 90         ActiveSupport::StringInquirer.new(ENV["RAILS_ENV"])
 91       end
 92     end

I thinks we should use RACK_ENV for usefulness and consistency.

[rafaelfranca]

Thanks