Fix warnings, redefined methods
Fix warnings, redefined methods

- test/test_memcached_store.rb:161: warning: method redefined; discarding old test_read_multi
- test/test_memcached_store.rb:144: warning: previous definition of test_read_multi was here
- test/test_memcached_store.rb:174: warning: method redefined; discarding old test_read_multi_with_expires
- test/test_memcached_store.rb:152: warning: previous definition of test_read_multi_with_expires was here

Shopify/memcached_store