bump postgres shared_buffers to 1Gb

* on postgres 9.2 we couldn't do this without changing some sysctl
  values and rebooting the server. Now that we're using 9.3, changing
  this only requires a postgres reload
* I haven't put any science into choosing 1Gb. The postgres docs suggest
  that on systems with plenty of RAM, a higher value might be suitable.
  Let's start with this (it's certainly better than 24MB!) and see how
  we go