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

requirements/edx/github.txt

@@ -22,4 +22,4 @@
 -e git+https://github.com/edx/django-waffle.git@823a102e48#egg=django-waffle
 -e git+https://github.com/edx/event-tracking.git@f0211d702d#egg=event-tracking
 -e git+https://github.com/edx/bok-choy.git@v0.1.0#egg=bok_choy
--e git+https://github.com/edx-solutions/django-splash.git@15bf143b15714e22fc451ff1b0f8a7a2a9483172#egg=django-splash
+-e git+https://github.com/edx-solutions/django-splash.git@6e8bd685214aebe78d0cfdf5572e27c5f1bc357a#egg=django-splash