Add process ID to server data

It's useful to report the PID of the process that raised the exception,
this commit adds it to server data so we can see it in Rollbar.

I will manually test it a little more, please don't merge until I report that it works properly.

I've only run the specs with this Ruby:
ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-linux]

Process.pid seems to work on one Windows computer we have here, though I don't know how universal support is or whether an exception is thrown if it is not supported. Ruby documentation says it doesn't work on all platforms.
In some other libraries (like pyrollbar) we send the process args as `server.argv`. Should probably stick with that here.