The original Vagrantfile was meant to be used with Vagrant 1.7.4, but
required 2 patches to Vagrant for proper functionality, which I had
submitted as pull requests. Vagrant has since released a new version,
1.8.0, which has updates the Salt provisioner. PR status:
 - [One PR](hashicorp/vagrant#6474) was merged
 - [Another PR](hashicorp/vagrant#6473) was not

The second PR was rejected "in favor" of [a third
PR](hashicorp/vagrant#6073) which simply removed
the config_dir option entirely instead of using sudo to be able to write
in a priveleged directory. [An unrelated
PR](https://github.com/mitchellh/vagrant/pull/6382/files) removed a
second option we had been using: install_command.

These changes in Vagrant were due to incompabilities with the official
Salt bootstrap script, so this commit brings the install_salt script's
behavior closer to that of the official Salt bootstrap script as well.
Namely, it can now copy over configuration files as part of
installation, using the same flags as the official script.

The configure_salt script is now used just to setup the salt and pillar
roots on Travis (this is accomplished via shared folders in Vagrant),
hence the renaming.

With this commit and the new Vagrant version, the included Vagrantfile
now works with a vanilla Vagrant 1.8.0 install, no patching needed.