Change default game but add "PRO" mutator to restore old rules.

Default game:
- CATs have 150 instead of 75 health but no damper.
- CATs don't have shields.
- Decreased falling damage.

New mutators:
- damper: CATs take less damage the more energy they have.
- halfhealth: CATs have 75 instead of 150 health.
- lowhealth: CATs have 50 instead of 150 health.
- crater: Triple falling damage.
- PRO: Combo: damper/shields/halfhealth/crater
Bump version to "p.5.4+dev2".