Changing enemy spawn

Fixed bug at gameover restart.

Heard that too many enemies spawn and are too fast, so I changed some behind the scenes things.

Enemies are now slightly slower and the boot has less impact.

Old enemy spawn was:  Number of Enemies = (Level+1)^2 + (Ratmeat_value * Level)

Now enemy spawn is:  Number of Enemies =  math.floor((Level/3)^1.5) + (Ratmeat_value * Level) + 3)

Should look more like this now.