Add warning to Vagrantfile for vm.host_name

[dkingofpa]

When setting config.vm.host_name in the Vagrantfile, there are certain characters that won't work. Add a short warning reminding the user of this potential gotcha. Perhaps it is just sub-domains that aren't allowed due to the VirtualBox, Vagrant, networking setup, et. al. components of our dev environment. In any case, here's an explanation of valid characters: http://en.wikipedia.org/wiki/Hostname#Restrictions_on_valid_host_names

For example, we'll be unable to resolve the url if we use a . in the host_name.

Bad host_name: test.example results in http://test.example.local being unresolvable.

Good host_name: test-example results in http://test-example.local being resolvable.