Update Vagrant version dependency.

A number of folks running older versions of vagrant experienced issues
getting nokogiri installed due to a bug that was fixed in vagrant
version 1.7.2: hashicorp/vagrant#3769

We shouldn't support older versions of Vagrant due to these issues.

README.md

@@ -4,7 +4,7 @@
 
 ------------
 * [virtualBox](https://www.virtualbox.org/wiki/Downloads) >= 4.3.x
-* [vagrant](http://downloads.vagrantup.com/) >= 1.6.x
+* [vagrant](http://downloads.vagrantup.com/) >= 1.7.2
 * [vagrant-hostmanager](https://github.com/smdahlen/vagrant-hostmanager)
   * Install with: `vagrant plugin install vagrant-hostmanager`