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 I noticed that a bunch of tests explicitly set `worker_processes  $MAX_THREADS` in the `setup.sh` command line for nginx.

I cargo cult-ed this code (meaning: I just copied the code without understanding it) - Would `auto` be better?

@hamiltont Sure seems like different suites use different values. Do you also gave a suggestion here?