Bug#915689: prevent from migrating to testing

[Michael Stone]

On Sun, Jan 20, 2019 at 03:25:12PM +0000, Thorsten Glaser wrote:
Michael Stone dixit:
No, that's something else that shouldn't have happened

It’s important to me because the upload of rng-tools (>> 2)
broke things on unstable.

So that should be fixed--the problem should not be made worse.

Please upload a fixed version of rng-tools instead, reverting the 
erroneous change.