exim4_4.50-6 is needed to work with libnss-ldap

[Jonathan Bastien-Filiatrault <joe@dastyle.net>]

Bug #299051 renders the MTA unusable and fills up the /var partition on 
a machine with NSS/LDAP setup. The testing version 4.50-4 is hit by that 
bug. It would greatly be appreciated if a release manager could bring 
version 4.50-6 into testing.

Please CC me.

Cheers,
Jonathan