Update vagrant-dev-env.rst

Fix some minor changes
ruby-dev package required for installing
vagrant-libvirt plugin.

TrivialFix

Change-Id: Id6d3552275ee28ab49804fe6cd1b00d2e8e3c1f4

doc/vagrant-dev-env.rst

@@ -42,7 +42,7 @@ On Fedora 22 it is as easy as::
 
 On Ubuntu 14.04 it is as easy as::
 
-    sudo apt-get install ruby-libvirt python-libvirt libvirt-dev nfs-kernel-server
+    sudo apt-get install vagrant ruby-dev ruby-libvirt python-libvirt libvirt-dev nfs-kernel-server
 
 **Note:** Many distros ship outdated versions of Vagrant by default. When in
 doubt, always install the latest from the downloads page above.