Use Haystack's SignalProcessors

In Haystack 2.0.0-beta, as of commit 041c480c8260dbe54feeeba52182f0788541d6b7, RealTimeSearchIndex is going away.

This commit follows the migration instructions to update to use
SignalProcessors instead of RealTimeSearchIndex.  The migration instructions
are at http://django-haystack.readthedocs.org/en/latest/migration_from_1_to_2.html#removal-of-realtimesearchindex

Addresses #570
Turn off Django debug mode for Travis tests

This is a workaround for toastdriven/django-haystack#716

Closes #570
Explicitly set django-notification version

Work around API change (see #571) until we can change code and test so
that we work with django-notification==1.0