#44 Handle destroyed machines in update_file

Keep track of machines that have been destroyed (they have a nil id).
Do not include them when updating the hosts file.

Since it is impossible to match on id, this is accomplished by matching
the exact ip / hostname / alias.  If any of these have been altered
since the entry was created the entry will not be removed from the hosts
file.

Potentially, this could be relaxed to only match on ip / hostname pairs.

lib/vagrant-hostmanager/hosts_file.rb

@@ -54,6 +54,7 @@ module VagrantPlugins
       def update_file(file)
         # build array of host file entries from Vagrant configuration
         entries = []
+        destroyed_entries = []
         ids = []
         get_machines.each do |name, p|
           if @provider == p
@@ -62,15 +63,21 @@ module VagrantPlugins
             id = machine.id
             ip = get_ip_address(machine)
             aliases = machine.config.hostmanager.aliases.join(' ').chomp
+            if id.nil?
+              destroyed_entries << "#{ip}\t#{host} #{aliases}"
+            else
               entries <<  "#{ip}\t#{host} #{aliases}\t# VAGRANT ID: #{id}\n"
               ids << id unless ids.include?(id)
             end
           end
+        end
 
         tmp_file = Tempfile.open('hostmanager', @global_env.tmp_path, 'a')
         begin
           # copy each line not managed by Vagrant
           File.open(file).each_line do |line|
+            # Eliminate lines for machines that have been destroyed
+            next if destroyed_entries.any? { |entry| line =~ /^#{entry}\t# VAGRANT ID: .*/ }
             tmp_file << line unless ids.any? { |id| line =~ /# VAGRANT ID: #{id}/ }
           end