Get rid of the bash-completion errors

[stevenrombauts]

As reported here, every ssh command that Vagrant runs spews out the following warnings:

INFO ssh: Execute: echo; printf $SSH_AUTH_SOCK (sudo=false)
DEBUG ssh: stderr: stdin: is not a tty
DEBUG ssh: stderr: Completion was not registered for box:
DEBUG ssh: stderr: The 'bash-completion' package is required but doesn't appear to be installed.