OOM when using Demobench + Explorer

By default we hardcode -XMX200m for explorer.jar which, after recent changes in 3.1 is not enough.
Steps to reproduce:
1. Build and run Demobench as described in repo (use 3.1 tag).
2. Start notary and two nodes
3. Once nodes are started, open Explorer on one of them.
4. Navigate to Cash, create new Issuance transaction - wait for disaster