Update ajv usage with latest cli version

[vdice]

The latest ajv cli that we use for json validation introduces some new flags/config that we'll need to update to get working again.

Sample output:

$ make validate
...
Testing json 'examples/101.01-bundle.json' against schema 'schema/bundle.schema.json'
strict mode: use allowUnionTypes to allow union type keyword at "https://cnab.io/v1/definitions.schema.json#" (strictTypes)
schema schema/bundle.schema.json is invalid
error: unknown format "uri-reference" ignored in schema at path "#/properties/%24id"
make: *** [validate] Error 1

I've played around with adding --allowUnionTypes which seems to work as intended, but I haven't been able to figure out the actual error around the unknown format "uri-reference" issue...

[carolynvs]

Are there any features in the newer version that we would like to use?