Travis, Tox: add Django 1.7 targets
Update Travis & Tox config

* Tox: wait for Solr to start before running tests
* Travis: allow solr & pip downloads to be cached
* Travis now uses start-solr-test-server.sh instead of travis-solr
* Test Solr configuration uses port 9001 universally as per the
  documentation
* Change start-solr-test-server.sh to change into its containing
  directory, which also allows us to remove the realpath dependency
* Test Solr invocation matches pysolr
    * Use get-solr-download-url script to pick a faster mirror
    * Upgrade to Solr 4.7.2
Tests: don't suppress Solr stderr logging

This will make easier to tell why Solr sometimes goes away on Travis
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