Update pursuit.service to max heap size of 3.5GB

[thomashoneyman]

The Pursuit server used to have 2GB of memory. We've been seeing regular crashes as the server runs out of memory. We've upgraded to a 4GB server and need to bump this compiler flag to take advantage of the increased memory available — otherwise we see errors like this one:

$ journalctl -tf pursuit.service
Dec 06 17:08:16 pursuit pursuit[1133]: Starting in production mode
Dec 06 17:08:16 pursuit pursuit[1133]: pursuit: Heap exhausted;
Dec 06 17:08:16 pursuit pursuit[1133]: pursuit: Current maximum heap size is 1610612736 bytes (1536 MB).
Dec 06 17:08:16 pursuit pursuit[1133]: pursuit: Use `+RTS -M<size>' to increase it.