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.

Fixes #359
Merge pull request #369 from jstrater/master

Changed the value used for $0 in subcommands