raise minimal Vagrant version to 1.5

Author: mkuzmin

lib/vSphere/plugin.rb

@@ -1,4 +1,14 @@
-require 'vagrant'
+begin
+  require "vagrant"
+rescue LoadError
+  raise "The Vagrant vSphere plugin must be run within Vagrant."
+end
+
+# This is a sanity check to make sure no one is attempting to install
+# this into an early Vagrant version.
+if Vagrant::VERSION < "1.5"
+  raise "The Vagrant vSphere plugin is only compatible with Vagrant 1.5+"
+end
 
 module VagrantPlugins
   module VSphere