Specify helper file via environment variable

In a very similar way to the one used to specify a port.

In our project, we need to maintain regular specs and integration specs completely separated (as in, two different `spec_helper`s that are never loaded at the same time, and running the specs in different processes). As spork hardcodes the path to `spec_helper`, we can not run a second spork server with the second helper file loaded instead. This change would allow it.