Set frozen_string_literal: true in colors.rb

We should use frozen string literals in this file. There are a bunch of
string literals in constants that should be constant.

Author: tenderlove

lib/thor/shell/color.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_relative "basic"
 require_relative "lcs_diff"