passing options instead of cb to spawnCommand

I was puzzled by why 'cb' was being passed to the spawnCommand method. I assume this was a typo? With the proposed change, you can now pass in options to the spawned command in order to do things like modify the PATH variable for the spawn'd process, etc.
Change every assertions to this structure: `sinon.assert.calledWithExactly(this.spawnCommandStub, 'nestedScript', ['install', 'path1', 'path2'], spawnEnv)`