Set variable to avoid getting stuck on prompt during setup.

Vagrantfile

@@ -40,6 +40,7 @@ Vagrant.configure("2") do |config|
     debconf-set-selections <<< 'mysql-server mysql-server/root_password_again password root'
 
     # Install package dependencies.
+    export DEBIAN_FRONTEND=noninteractive
     apt-get update
     apt-get install -y git zip unzip apache2 default-jdk mysql-server
     apt-get install -y libapache2-mod-php php-mbstring php-pear php php-dev php-gd php-intl php-json php-ldap php-mysql php-soap php-xml php-curl