updated method get in roster (roster[key] is 'userId' instead of attr…

[Vladlukhanin]

Make sure that you wrote the tests for your proposed changes and the existing test was success.

Made/Proposed changes:

• roster[key] is 'userId' instead of attr 'name' (roster keys must be user ids, not userId + appId);
• updated test for method 'get' in Roster;
• changed sdk version to 2.3.1.

How should this be manually tested?

• run jasmine test.

Does the documentation need an update?

• no.