Remove unused variable in specs

spec/rackstash/buffer_stack_spec.rb

@@ -69,7 +69,7 @@ describe Rackstash::BufferStack do
     end
 
     it 'always returns nil' do
-      new_buffer = stack.push
+      stack.push
       expect(stack.flush_and_pop).to be nil
       expect(stack.flush_and_pop).to be nil
     end

spec/rackstash/logger_spec.rb

@@ -202,11 +202,8 @@ describe Rackstash::Logger do
     end
 
     it 'formats the message' do
-      time = Time.now
       formatter = double('formatter')
-
       logger.formatter = formatter
-
       expect(formatter).to receive(:call)
         .with('DEBUG', instance_of(Time), Rackstash::PROGNAME, 'Hello World')