Player blue and red are not currently symmetrical

In https://github.com/alshedivat/lola/blob/master/lola/envs/coin_game.py

The symmetry is broken in favor of player red.
When the two players move at the same time on the cell with the coin, player red has the advantage to pick the coin (always pick before player blue)

In my implementation (where I do not use batch):
Currently we have:
```
if self.red_coin:
if self._same_pos(self.red_pos, self.coin_pos):
generate = True
reward_red = 1
reward_blue = 0
elif self._same_pos(self.blue_pos, self.coin_pos):
generate = True
reward_red = -2
reward_blue = 1
else:
reward_red = 0
reward_blue = 0

else:
if self._same_pos(self.red_pos, self.coin_pos):
generate = True
reward_red = 1
reward_blue = -2
elif self._same_pos(self.blue_pos, self.coin_pos):
generate = True
reward_red = 0
reward_blue = 1
else:
reward_red = 0
reward_blue = 0
```
To have the symmetry between red and blue, this should be changed to:
```
if self.red_coin:
if self._same_pos(self.red_pos, self.coin_pos) and self._same_pos(self.blue_pos, self.coin_pos):
if np.random.randint(0, 2):
generate = True
reward_red = 1
reward_blue = 0
else:
generate = True
reward_red = -2
reward_blue = 1
elif self._same_pos(self.red_pos, self.coin_pos):
generate = True
reward_red = 1
reward_blue = 0
elif self._same_pos(self.blue_pos, self.coin_pos):
generate = True
reward_red = -2
reward_blue = 1
else:
reward_red = 0
reward_blue = 0

else:
if self._same_pos(self.red_pos, self.coin_pos) and self._same_pos(self.blue_pos, self.coin_pos):
if np.random.randint(0, 2):
generate = True
reward_red = 1
reward_blue = -2
else:
generate = True
reward_red = 0
reward_blue = 1
elif self._same_pos(self.red_pos, self.coin_pos):
generate = True
reward_red = 1
reward_blue = -2
elif self._same_pos(self.blue_pos, self.coin_pos):
generate = True
reward_red = 0
reward_blue = 1
else:
reward_red = 0
reward_blue = 0
```

I can do a push request if needed.