OptaWeb Employee Rostering: Increase JVM heap memory

The application becomes unresponsive after being used for a short time
when relying on the default memory settings. I even saw it crash with
OutOfMemoryError once.

Increasing the heap memory size addresses these issues. I used the
application with these settings for some time and the memory consumption
settled down on 1.4 Gi so I assume 2 Gi should be enough (unless there's
a memory leak).