Tweak default gradle options

[ErikSchierboom]

If I look at the gradlew file I see:

DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'

To me, this looks like the exercises by default run with 64MB of memory. If that is correct, we should probaby update to a more reasonable amount.

[emiliolg]

AFAIK this are the options to run grade itself.
Gradle forks a new JVM to run the tests.