Release notifications 0.6.0

[jnweiger]

Reason

• Always return an int from Symfony Command execute method #376

[jnweiger]

https://github.com/owncloud/notifications/releases/download/v0.6.0-rc.1/notifications-0.6.0-rc.1.tar.gz
https://github.com/owncloud/notifications/releases/download/v0.6.0-rc.2/notifications-0.6.0-rc.2.tar.gz

[jnweiger]

Issues seen while testing

• [QA] notifications.svg could not be loaded when app is in apps-external #382