cap sensitive unix based fix

Linux systems are cap sensitive 
```
metulburr@arch ~/Downloads/bubbleshooter-master $ python2 puzzlebobble.py 
Traceback (most recent call last):
  File "puzzlebobble.py", line 665, in <module>
    main()
  File "puzzlebobble.py", line 165, in main
    score, winorlose = runGame()
  File "puzzlebobble.py", line 182, in runGame
    arrow = Arrow()
  File "puzzlebobble.py", line 107, in __init__
    arrowImage = pygame.image.load('arrow.png')
pygame.error: Couldn't open arrow.png

```