Travis: wait for Solr to start before running tests

Closes #1040
Refactor get_model_ct to handle Django 1.7, add tuple version

We have a mix of model _meta access which usually expects strings but in
a few places needs raw values. This change adds support for Django 1.7
(see https://code.djangoproject.com/ticket/19689) and allows raw tuple
access to handle other needs in the codebase
Consolidate model meta references to use get_model_ct (see #981)

This use of a semi-public Django interface will break in Django 1.7
and we can start preparing by using the existing
haystack.utils.get_model_ct function instead of directly accessing
it everywhere.
Haystack admin: refactor to use get_model_ct_tuple
Refactor simple_backend to use get_model_ct_tuple
Use get_model_ct_tuple to generate template name