Automatically update version numbers of newly added @since tags

[TimvdLippe]

When we develop a new API, we add @since to note in which version this is released. However, we do not know what specific version it will be, as Shipkit handles that. In other words, I would like 2 improvements:

1. Add possibility to specify@since NEXT_VERSION, which Shipkit (as part of the release process) updates to the actual version number
2. (Optional) Add a check that every new API has this tag.

Previously filed as https://github.com/mockito/mockito/issues/697