Delete warning in mruby test code

Before:
```
/Users/hiroejun/work/mruby-redis/mruby-redis/test/redis.rb:122:17: ambiguous first argument; put parentheses or even spaces
CC    build/host/test/no_mrb_open_test.c -> build/host/test/no_mrb_open_test.o
AR    build/host/test/mrbtest.a 
LD    build/host/test/mrbtest 
>>> Test host <<<
mrbtest - Embeddable Ruby Test
```