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
This line should go [here](https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/toolset/setup/linux/prerequisites.sh#L23), with a comment explaining what is being installed