Raise an error on RealtimeUpdates::validate_update if @secret is not set

RealtimeUpdates::validate_update requires @secret to be set in the RealtimeUpdates
instance.

But the RealtimeUpdates can be initialized without it (with @app_access_token instead),
in which case validate_update will break.
Bump version to 1.7.0
eerge branch 'master' of github.com:arsduo/koala
Add test for AppSecretNotDefined raising
Merge branch 'theosp-realtime_updates_validate_update_bug_fix'
Update changelog
Merge branch 'master' of github.com:arsduo/koala
Fix stray stub usage from recent pull request