Fix % bug

if you try to order a by similar distance against a query with the symbol `%` in it, an `IndexError` is raised. 
I find this ticky error through Sentry, here: https://github.com/mgaitan/preciosa/issues/190

reference http://python.6.x6.nabble.com/Escape-percent-sign-in-QuerySet-extra-td4998097.html