remove custom `OMNIBUS_INSTALL_URL` now that the official install.sh sup...

This is a precautionary removal of the `OMNIBUS_INSTALL_URL` env var in `W:\home\.vagrant.d\Vagrantfile` now that the official install.sh supports caching (see opscode/opscode-omnitruck#33)

It is not effective yet until a new version of vagrant-omnibus is released which makes use of the `-d` flag. Track schisamo/vagrant-omnibus#68 for further progress on this.

The decision to remove the currently set `OMNIBUS_INSTALL_URL` env var eagerly is that we don't want to release this as 1.0. Until the above PR is ready, the existing caching of the omnibus packages will not work anymore. If you still need it you can always add it back manually.