Updated HACKING.md to more closely resemble reality

HACKING.md is out-of-date.

* It said securedrop relies on precise, but now it relies on trusty
* Vagrant technically starts two VMs, not one
* We require vagrant-hostmanager and ansible to be installed to work at all, but this wasn't in the instructions
* I also fixed some formatting
@dolanjs Do we even need this step? Won't Vagrant do this for us automatically the first time someone calls `vagrant up`?