References chapter subheader fix for psychology and across texts

Fixes #438 
For this pull request, pursuant to our discussion this morning w/Phil, Ed, Derek, I investigated the issue further.  Although the current issue we talked about still exists, and going forward I will place these types of things in the book.less file I think I have a solution for this one that's elegant.

Looking at all our books, I see that this subheading under references (Chapter 1, Chapter 2) is always in Serif font. For the chapter-area namespace in precalc, there isn't a font-family property, it simply inherits the body font which is Sans-Serif

My solution is here to add the font-family to the precalculus-slot,  then I also found in history-skeleton a reference to (#part>#book-end>#end-of-book-references>.style();) which can be removed.  By placing the font declaration in precalc-slots, in effect we allow it the other books to inherit this.  At this point if we need to change this we can alter in history/econ slot.  

This would be an override which is ok (additive) and much better than a reset (subtractive)  For each fix, I will look at this way of doing things.... all roads leading to precalculus as they say.