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