debian: strictly depend on the same version of libnotmuch

It took quite some time to debug why folder: searches didn't work for me
though I had notmuch 0.6~rc1 installed. amdragon in #notmuch found out
that I still had libnotmuch1 0.5+nmu3 installed.

To prevent the same problem in the future let notmuch depend on the same
version of libnotmuch1.

Reviewed-By: David Bremner <david@tethera.net>
  The underlying issue is that the libnotmuch interface is not
  entirely captured by the set of exported symbols. In particular the
  query syntax can change without being visible to the linker at all.