update Gemfile and test Vagrantfile to support vagrant 1.5

relates to #80

Gemfile

@@ -1,7 +1,9 @@
 source 'https://rubygems.org'
 
 group :development do
-  gem 'vagrant', :git => 'git://github.com/mitchellh/vagrant.git', :tag => 'v1.2.7'
+  gem 'vagrant', :git => 'git://github.com/mitchellh/vagrant.git', :tag => 'v1.5.1'
 end
 
-gemspec
+group :plugins do
+  gemspec
+end

test/Vagrantfile

@@ -1,7 +1,9 @@
 # -*- mode: ruby -*-
 # vi: set ft=ruby :
 
-Vagrant.require_plugin('vagrant-hostmanager')
+if Gem::Version.new(::Vagrant::VERSION) < Gem::Version.new('1.5')
+  Vagrant.require_plugin('vagrant-hostmanager')
+end
 
 Vagrant.configure('2') do |config|
   config.vm.box = 'precise64'