splash-url-exclusion: Exclude '/heartbeat' from redirect

Update the dependency to load this commit:
https://github.com/edx-solutions/django-splash/commit/6e8bd685214aebe78d0cfdf5572e27c5f1bc357a

Which allows to prevent `/heartbeat` from being redirected, which is necessary on production servers as the ELB can't set a cookie or be logged in.