Release 0.6.20

Release notes 0.6.20: -Add a pickup sound when the player collects dropped cash -Fixed AUTO SENTRY's bullets being visible through walls -Fixed some cases of zombies doing damage to buildings from long distances -Increase ELECTRIC GRIDs damage to zombies by 200% -Increase SPIKE BARRICADE and WOODEN SPIKES unupgraded damage -Increase unupgraded buildings' health -Increased TELEPORTER health by 250% -Increased TERMINATOR TOWER health by 200% -Reduce HELL DOGS damage towards players by 50% -Reduce HELL DOGS explosive damage by 40% -Reduce TERMINATOR TOWER charging time by 50% -Reduce how much BARBED WIRE slows down the player -Reduce the placement area size of the FLAMEABLE BARRELS, EXPLOSIVE BARRELS and LAND MINES take significantly -Reduce SPIKE BARRICADE and WOODEN SPIKES limit by 40% -Revert to LASER FENCEs doing instant damage