Delete && Up didn't work properly

Because the "Vagrant: Destroy && Up" plugin command ran both 'vagrant destroy' and 'vagrant up' asynchronously, they didn't always happen in the right order (and probably ran simultaneously.)

Changed the code to run 'destroy' synchronously, and only run 'up' afterwards if destroy returned an errorcode of 0 (i.e. no errors)

This shouldn't be too much of a performance issue because destroy takes very little time to complete and vagrant up, which can take forever, is still run asynchronously.