Simplify cygwin check everywhere

bin/vagrant

@@ -36,14 +36,8 @@ if ARGV.include?("--no-color")
   opts[:ui_class] = Vagrant::UI::Basic
 elsif !Vagrant::Util::Platform.terminal_supports_colors?
   opts[:ui_class] = Vagrant::UI::Basic
-elsif !$stdout.tty?
-  # If we're not a TTY, verify we're not in Cygwin. Cygwin always
-  # reports that stdout is not a TTY, when in fact it is.
-  ENV["VAGRANT_DETECTED_OS"] ||= ""
-
-  if !ENV["VAGRANT_DETECTED_OS"].downcase.include?("cygwin")
+elsif !$stdout.tty? && !Vagrant::Util::Platform.cygwin?
     opts[:ui_class] = Vagrant::UI::Basic
-  end
 end
 
 # Default to colored output

lib/vagrant/util/platform.rb

@@ -17,6 +17,9 @@ def leopard?
         end
 
         def cygwin?
+          return true if ENV["VAGRANT_DETECTED_OS"] &&
+            ENV["VAGRANT_DETECTED_OS"].downcase.include?("cygwin")
+
           platform.include?("cygwin")
         end
 
@@ -67,8 +70,7 @@ def platform_path(path)
         # output.
         def terminal_supports_colors?
           if windows?
-            return ENV.has_key?("ANSICON") ||
-              ENV["VAGRANT_DETECTED_OS"].downcase.include?("cygwin")
+            return ENV.has_key?("ANSICON") || cygwin?
           end
 
           true