Improves handling of file:// URLs.

Vagrant uses file URLs (for example "file:///X:/98_Transfer/fsr/boxes/ad.box") to address boxes on Windows network drives. The vagrant-box-updater can not handle these file URLs as Ruby's File.mtime does not accept file URLs. This commit converts the file URL to an valid string that can be used by File.mtime.
Gives the user the power to decide when to apply an box update.

Changes the update progress to ask the user to apply an available update until the update is applied (for every time the user calls 'vagrant up'). So far the vagrant-box-updater just asked once and if the user denies updating, the updater keeps track of this decision and ignores this update until another update is available. The auto-update behaviour is not affected by this commit.