Update box version string so the OS doesn't influence sorting

[ksclarke]

It seems vagrant is using version string rather than timestamp to pull updated versions of the box. Our current versioning string (e.g., v0.3-centos-SNAPSHOT-1537418555) could cause problems if there is ever an Ubuntu version (I know we're not going to run Ubuntu at UCLA, but just generally speaking). A low priority fix is to update the version string format to: v0.3-SNAPSHOT-1537418555-centos. This allows the timestamp to sort before the OS.

This will be a personal time fix one evening. It's not a priority for UCLA.