Find and install box file for multi-provider boxes automatically
Changed the provider name from "vSphere" to "vsphere".

  Lower-case is required by Vagrant when a box is not installed locally, and it's downloaded from a remote URL.
Fixed broken tests
Semver
Merge pull request #87 from mkuzmin/provider-name

Lower-case provider name (vSphere=>vsphere)
Use bumpversion for handling semver.