xapi-schema update

[kelvinqian00]

Update to the latest xapi-schema dep and add tests for empty maps.

As a demonstration, edit dev-resources/statements/long.json so that the verb property is as follows:

"id": "http://adlnet.gov/expapi/verbs/attended",
"display": {}

Then store the statement:

% curl -X POST -H "X-Experience-API-Version:1.0.3" -H "Content-Type:application/json" -d @dev-resources/statements/long.json http://0.0.0.0:8080/xapi/statements
["6690e6c9-3ef0-4ed3-8b37-7f3964730bee"]%

And retrieve it:

% curl -X GET -H "X-Experience-API-Version:1.0.3" -H "Content-Type:application/json" http://0.0.0.0:8080/xapi/statements
{"statements":[{"verb":{"id":"http://adlnet.gov/expapi/verbs/attended","display":{}}, ...}]}

[kelvinqian00]

Closed because of https://github.com/yetanalytics/xapi-schema/pull/72