Revert keystone support in v3 or merge keystone support in iron_core_php

[mike182uk]

Hey,

I'm getting Notice: Undefined property: IronMQ::$use_keystone when using the v3 branch.

The reason i'm getting this error is because iron-io/iron_core_php#12 has not been merged but #38 has been merged. IMO both of these PR's should of been merged at the same time as they are dealing with the same bit of functionality.

I noticed a fix for this was recently merged (#42) but i don't think this is the correct way to handle the problem. When iron-io/iron_core_php#12 finally gets merged there is going to be a redundant isset($this->use_keystone) which will most likely get forgot about and left in.

A better approach may be to either get iron-io/iron_core_php#12 merged and revert #42 or revert #38 and #42 until iron-io/iron_core_php#12 can be merged?