Skill book focus rebalance

Reduce focus loss from skill books from -100 to -50.  Had this fix as part of my studious nature perk but my total inability to get gettext to work has essentially cut me out of development :(   I'm not going to be able to finish studious nature, so I thought I'd just submit this by itself.  Talked with Funnyman about this when I was still able to work on my perk mod, and we seemed to think this value was pretty balanced (Even Funnyman agreed the original value was WAY to high).  It worked fine before with all my other changes, so I don't see any reason it
shouldn't work on its own.  Although for obvious reasons I can't test it.  Well this'll probably be my last pull request so I wish you all the best with working on this awesome game.  I'll probably still play the experimental builds occasionally but I can't see myself trying to code anything new since I can't test it.

So see ya chummers, G'luck in your struggles!  (Hope someone gets this reference)