Use a proper feature check to check if Data is defined

Author: eregon

lib/pp.rb

@@ -438,7 +438,7 @@ def pretty_print(q) # :nodoc:
   def pretty_print_cycle(q) # :nodoc:
     q.text sprintf("#<data %s:...>", PP.mcall(self, Kernel, :class).name)
   end
-end if "3.2" <= RUBY_VERSION
+end if defined?(Data.define)
 
 class Range # :nodoc:
   def pretty_print(q) # :nodoc:

test/test_pp.rb

@@ -143,7 +143,7 @@ def test_struct
     assert_equal("#{a.inspect}\n", PP.pp(a, ''.dup))
   end
 
-  if "3.2" <= RUBY_VERSION
+  if defined?(Data.define)
     D = Data.define(:aaa, :bbb)
     def test_data
       a = D.new("aaa", "bbb")