Godeps: Include github.com/xeipuuv/gojsonschema

Generated with:

  $ godep save ./...

When I originally did this with v74 I needed to move the entries from
Godeps/_workspace/src to vendor/ by hand, but now that I'm using godep
v77 they're added to vendor/ automatically.

I'm not sure why github.com/stretchr/testify/assert and descendants
weren't added to Godeps.json back in 15577bd (add runtime struct; add
create test, 2017-08-24, opencontainers#447), since that's when they landed in
vendor/.  The fact that they weren't there means it's hard to tell
whether the changes my godep call made are moving the libraries
forward or backward in time, but I expect they're moving forward
because I just updated them with 'go get -u ...'.

Signed-off-by: W. Trevor King <wking@tremily.us>