Vagrantfile: update base image version to centos 7.6

Author: tierpod

Vagrantfile

@@ -2,7 +2,7 @@
 # vi: set ft=ruby :
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "bento/centos-7.5"
+  config.vm.box = "bento/centos-7.6"
 
   # config.vm.network "forwarded_port", guest: 80, host: 8080
   config.vm.network "private_network", ip: "192.168.33.21"