Use curl instead of wget in Vagrantfile

Use curl instead of wget in Vagrantfile to fetch the install script due
to a bug in the default version of wget on Debian Squeeze.

Fixes https://github.com/mysociety/alaveteli/issues/1691

> wget debian packages prior to 1.14-1 don't have SNI [1] support so the
> webserver don't know which ssl certificate needs to use
>
> – https://github.com/Hexxeh/rpi-update/issues/65#issuecomment-13332143