Replace `attr_accessor` with instance variable

The `features` hash does not have to be accessed from outside the
repository class.

lib/flop/repository/memory.rb

@@ -1,18 +1,16 @@
 module Flop
   module Repository
     class Memory
-      attr_accessor :features
-
       def initialize
-        self.features = Hash.new(false)
+        @features = Hash.new(false)
       end
 
       def get(name)
-        features[name]
+        @features[name]
       end
 
       def set(name, value)
-        features[name] = value
+        @features[name] = value
       end
     end
   end