Fix arguments

The following func has too many arguments:
```
mrb_value mrb_redis_zrange(mrb_state *mrb, mrb_value self, const char *cmd)
mrb_value mrb_redis_zrevrange(mrb_state *mrb, mrb_value self, const char *cmd)
```

It is not necessary to have cmd arguments:
```
mruby-redis/src/mrb_redis.c:381: warning: passing argument 4 of ‘mrb_define_method’ from incompatible pointer type
mruby-redis/src/mrb_redis.c:382: warning: passing argument 4 of ‘mrb_define_method’ from incompatible pointer type
```
They are correct arguments:
```
mrb_value mrb_redis_zrange(mrb_state *mrb, mrb_value self)
mrb_value mrb_redis_zrevrange(mrb_state *mrb, mrb_value self)
```