Fix mrb_intern because of too few arguments

Fix the following build errors:
```
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c: In function ‘mrb_redis_get_context’:
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c:61: error: too few arguments to function ‘mrb_intern’
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c: In function ‘mrb_redis_connect’:
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c:81: error: too few arguments to function ‘mrb_intern’
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c: In function ‘mrb_redis_close’:
/Users/hiroejun/work/mruby-redis/mruby-redis/src/mrb_redis.c:376: error: too few arguments to function ‘mrb_intern’
```