Refactor DevKit valid compilers checking

Thanks to @Azolo aka Justin Baker for suggesting this refactor.

config/devkit.rb

@@ -1,23 +1,8 @@
 require 'ostruct'
 
 module DevKitInstaller
-
   DEFAULT_VERSION = 'tdm-32-4.5.2'
   COMPILERS = {}
-
-  # TODO update this list when adding any new compiler or compiler version!
-  #      The format is a String consisting of vendor-bits-version
-  VALID_COMPILERS = [
-    'tdm-32-4.6.1',
-    'tdm-32-4.5.2',
-    'tdm-64-4.6.1',
-    'mingw-32-4.6.2',
-    'mingw-32-4.5.2',
-    'mingw-32-3.4.5',
-    'mingw64-32-4.6.3',
-#    'mingw64-64-4.4.5',
-  ]
-
 end
 
 # load DevKit compiler definitions

recipes/devkit/devkit.rake

@@ -36,7 +36,7 @@ namespace(:devkit) do
 
   # canonicalize DevKit compiler version and check if version is supported
   ENV['DKVER'] = ENV['DKVER'].nil? ? DevKitInstaller::DEFAULT_VERSION.downcase : ENV['DKVER'].downcase
-  fail '[FAIL] invalid DKVER value provided' unless DevKitInstaller::VALID_COMPILERS.include?(ENV['DKVER'])
+  fail '[FAIL] invalid DKVER value provided' unless DevKitInstaller::COMPILERS.has_key?(ENV['DKVER'])
 
   task :installer, [:target] => [DevKitInstaller::DevKit.inno_config, :innosetup] do |t, args|
     InnoSetup.iscc(DevKitInstaller::DevKit.inno_script,