Increase max JVM heap size in `gradle.properties` (#1373)

It's currently defaulting to 512 MiB. I often see this message in my
build logs:

> Daemon will be stopped at the end of the build after running out of JVM memory

I've done some (unscientific) profiling with this setting enabled, and
max heap size can peak at ~600 MiB, so I see no reason to increase it
more than 1024 MiB.