simplify `Vagrantfile`.

Vagrantfile

@@ -11,11 +11,7 @@ Vagrant.configure("2") do |config|
    config.vm.provider "virtualbox" do |vb, override|
       #vb.gui = true
       vb.memory = "2048"
-      if RbConfig::CONFIG['host_os'] =~ /mswin|msys|mingw|bccwin|wince|emc/
-         override.vm.box = "debian/contrib-jessie64"
-      else
-         override.vm.box = "debian/jessie64"
-      end
+      override.vm.box = "debian/contrib-jessie64"
    end
 
    # the docker provider is preferable to the virtualbox provider,
@@ -24,6 +20,8 @@ Vagrant.configure("2") do |config|
       override.vm.box = "tknerr/baseimage-ubuntu-16.04"
    end
 
+    config.vm.network "forwarded_port", guest: 8888, host: 8888, auto_correct: true
+
    config.vm.provision "shell", inline: <<-SHELL
       /bin/sh /vagrant/scripts/setup/vagrant.sh
    SHELL