Consistently use instances instance variable in Mockery

Previously we were using a mixture of the instance variable and the
Mockery.instance method.

Now we initialize the instances instance variable to an empty array on
the first call to setup since that should be the first place that
references the instance variable.

Author: nitishr

lib/mocha/mockery.rb

@@ -35,12 +35,13 @@ def raise_not_initialized_error
 
     class << self
       def instance
-        instances.last || Null.new
+        @instances.last || Null.new
       end
 
       def setup
+        @instances ||= []
         mockery = new
-        mockery.logger = instance.logger unless instances.empty?
+        mockery.logger = instance.logger unless @instances.empty?
         @instances.push(mockery)
       end
 
@@ -53,12 +54,6 @@ def teardown
       ensure
         @instances.pop
       end
-
-      private
-
-      def instances
-        @instances ||= []
-      end
     end
 
     def named_mock(name)