add ruby/ngx_mruby framework

Hello,

I was able to get this installed and running in a vagrant dev env.
It does exit after running the test, so there may be a bug in my code or the current suite.

Any pointers would be appreciated.

thanks
@matsumoto-r Does `db.execute()` runs without blocking nginx (like lua)?
If it does, I think workers=auto may be better.
If it doesn't, I think workers=threads*2~3 is better.

@hamiltont Can you use placement group on EC2 for low and stable network latency?