Consistently use start-solr-test-server

This avoids an outside dependency and allows us to use Travis' 
download caching

* Travis now uses start-solr-test-server.sh
* Travis can cache the Solr downloads
* Change start-solr-test-server.sh to accept a BACKGROUND_SOLR env
  variable to control whether it backgrounds Solr
* Change start-solr-test-server.sh to change into its containing
  directory, which also allows us to remove the realpath dependency
* Everything should now use port 9001 as per the documentation