Fix docs link for email backends
Fix docs link for email backends

This link is broken. I've updated it to point to version 1.7 even though
Sentry is using Django 1.6. The Django 1.6 docs don't seem to be online
anymore.

BuildingLink/sentry,ifduyue/sentry,mvaled/sentry,mitsuhiko/sentry,BuildingLink/sentry,mvaled/sentry,JackDanger/sentry,BuildingLink/sentry,ifduyue/sentry,fotinakis/sentry,zenefits/sentry,BuildingLink/sentry,gencer/sentry,fotinakis/sentry,beeftornado/sentry,gencer/sentry,looker/sentry,jean/sentry,JamesMura/sentry,jean/sentry,alexm92/sentry,ifduyue/sentry,beeftornado/sentry,JamesMura/sentry,looker/sentry,mvaled/sentry,jean/sentry,zenefits/sentry,fotinakis/sentry,mitsuhiko/sentry,zenefits/sentry,gencer/sentry,zenefits/sentry,JamesMura/sentry,alexm92/sentry,zenefits/sentry,looker/sentry,beeftornado/sentry,ifduyue/sentry,alexm92/sentry,jean/sentry,ifduyue/sentry,mvaled/sentry,fotinakis/sentry,BuildingLink/sentry,gencer/sentry,JackDanger/sentry,JamesMura/sentry,mvaled/sentry,gencer/sentry,JackDanger/sentry,mvaled/sentry,looker/sentry,JamesMura/sentry,looker/sentry,jean/sentry