Changed the value used for $0 in subcommands

$0 should be the first element of argv, which is generally the name of the program being run. Right now Spring is including the rest of argv as well. This is a problem for RSpec, which varies its behavior based on $0.

There's a lot more information on this issue [here](/jonleighton/spring-commands-rspec/issues/21).

@jonleighton suggested fixing spring instead of spring-commands-rspec, and he also wrote the original code that this patch modifies, so this seems like a shoo-in.

Fixes #359