Replace Mailcatcher with Mailhog in `README.md`.
Replace Mailcatcher with Mailhog in `README.md`.

New installations of VVV will install Mailhog, not Mailcatcher. The
project's README file should reflect this.

DrewAPicture/varying-vagrant-vagrants,trepmal/varying-vagrant-vagrants,speartail/VVV,speartail/VVV,Varying-Vagrant-Vagrants/VVV,saucal/VVV,DrewAPicture/varying-vagrant-vagrants,saucal/VVV,Varying-Vagrant-Vagrants/VVV,saucal/VVV,trepmal/varying-vagrant-vagrants,Varying-Vagrant-Vagrants/VVV