Update Vagrantfile to be more resilient

This changeset adds an explicit Vagrant version constraint (because of
the Vagrant Cloud box reference) and updates the mechanism to pull down
dependent Ansible roles.

Attempts to resolve: #38
Merge pull request #40 from OpenTreeMap/hmc/vagrant-version-constraint

Update Vagrantfile to be more resilient
Read proxy address from an environment variable

The previous version would fail if the vagrant-proxyconf plugin was
installed but a proxy was not running.
Remove no longer used vagrantfile helper function
Merge pull request #39 from jwalgran/topic/pass-proxy-as-env-var

Read proxy address from an environment variable
Ensure that Vagrant functions in directories relative to root

Certain paths in the Vagrantfile were set relative to the current
working directory. Now they are relative to the Vagrantfile itself. This
allows the `vagrant` command to work from subdirectories within the
project.

Attempts to resolve: #41
Merge pull request #42 from OpenTreeMap/hmc/vagrant-relative

Ensure that Vagrant functions in directories relative to root