Add missing version guard for the Kernel#pp spec.

Author: nirvdrum

spec/ruby/core/kernel/pp_spec.rb

@@ -1,9 +1,11 @@
 require_relative '../../spec_helper'
 
-describe "Kernel#pp" do
-  it "lazily loads the 'pp' library and delegates the call to that library" do
-    # Run in child process to ensure 'pp' hasn't been loaded yet.
-    output = ruby_exe("pp [1, 2, 3]")
-    output.should == "[1, 2, 3]\n"
+ruby_version_is "2.5" do
+  describe "Kernel#pp" do
+    it "lazily loads the 'pp' library and delegates the call to that library" do
+      # Run in child process to ensure 'pp' hasn't been loaded yet.
+      output = ruby_exe("pp [1, 2, 3]")
+      output.should == "[1, 2, 3]\n"
+    end
   end
 end