add ctf version to metadata

[clarkfitzg]

The metadata files are going to evolve as we use and develop them, so we should add version information for compatibility. This will allow us to read different versions of ctf data. This should be as simple as adding the following to the metadata:

version: 0.1.0

[clarkfitzg]

I wonder if we could get by without needing a version. Could we put everything required to specify a version in the @context or some other machine readable place?