Switch Vagrant to PHP 7.2
As WMF production's switch to it approaches, we need to start using
it for development.
- 3 mail-related packages were not available for 7.2. 2 of them are now installed via Composer:
- php7-net-smtp and php7.2-auth-sasl was optional, as evidenced by production working without it.
- php7.2-igbinary is required by redis.so