Run build script using login shell

I think the build script has always been intended to be run using a login shell, however, due to the way this is run it's not run in a login shell, which means that RVM isn't sourced unless you add it to some other places (which is done on the Linux VMs but not the OS X VMs). The OS X worker is currently running with this, I think it should be fine to deploy to Linux as well, although we probably want to test this on staging first.