Make PStore support as optional

Author: hsbt

lib/yaml/store.rb

@@ -3,7 +3,11 @@
 # YAML::Store
 #
 require 'yaml'
-require 'pstore'
+
+begin
+  require 'pstore'
+rescue LoadError
+end
 
 # YAML::Store provides the same functionality as PStore, except it uses YAML
 # to dump objects instead of Marshal.
@@ -83,4 +87,4 @@ def empty_marshal_data
   def empty_marshal_checksum
     CHECKSUM_ALGO.digest(empty_marshal_data)
   end
-end
+end if defined?(::PStore)

test/yaml/test_store.rb

@@ -177,4 +177,4 @@ def test_with_options
     end
     assert_equal(indentation_3_yaml, File.read(@yaml_store_file), bug12800)
   end
-end
+end if defined?(::YAML::Store)