Fix debian builds by correcting debian/changelog

Remove 1.8 debian/changelog entry with a date of "TBD" as this causes
the debian build process to fail.  We can add this back with the 1.8
release becomes a reality, or add it back now with a valid date in the
future.

packaging/debian/changelog

-ansible (1.8) unstable; urgency=low
-
-  * 1.8 release (PENDING)
-
- -- Michael DeHaan <michael@ansible.com>  TBD
-
 ansible (1.7) unstable; urgency=low
 
   * 1.7.0 release