Enable ssh-agent forwarding

@craigschneider @ChrisPetr0 @aaronFranssell 
I have made a minor change to the Vagrantfile to enable ssh-agent forwarding which should allow the user to use the ssh-agent/ssh keys on the host machine instead of needing to set up ssh keys inside the VM for things like git.

It is currently working great on my machine, being as it's a setting there's not a lot to test.