Switch to redis-rb@5

`redis.evalsha` just returns the script result now, not the SHA. So we
instead check the SHA from the `redis.load` command.

Note this no longer supports redis-rb@4

redis_scripts.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |gem|
   gem.files         = `git ls-files`.split("\n")
   gem.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
 
-  gem.add_runtime_dependency 'redis', '~> 4.0'
+  gem.add_runtime_dependency 'redis', '~> 5.0'
   gem.add_development_dependency 'temporaries', '~> 0.3.0'
   gem.add_development_dependency 'ritual', '~> 0.4.1'
 end