Update comment in Vagrantfile.
Update comment in Vagrantfile.

Make it clear that the Vagrantfile is for developing Passenger itself,
not for apps.

[ci:skip]

clemensg/passenger,antek-drzewiecki/passenger,kewaunited/passenger,clemensg/passenger,phusion/passenger,kewaunited/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,clemensg/passenger,kewaunited/passenger,phusion/passenger,clemensg/passenger,cgvarela/passenger,cgvarela/passenger,cgvarela/passenger,kewaunited/passenger,clemensg/passenger,clemensg/passenger,kewaunited/passenger,cgvarela/passenger,phusion/passenger,phusion/passenger,kewaunited/passenger,clemensg/passenger,phusion/passenger,kewaunited/passenger,phusion/passenger,clemensg/passenger,cgvarela/passenger,phusion/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,cgvarela/passenger,antek-drzewiecki/passenger,cgvarela/passenger,cgvarela/passenger,kewaunited/passenger,phusion/passenger