Update the sr.wikibooks logo.
svg: https://commons.wikimedia.org/wiki/File:Wikibooks-logo-sr-alternate.svg
1x png: https://commons.wikimedia.org/wiki/File:Wikibookslogo-sr-p-1x.png
1.5x png: https://commons.wikimedia.org/wiki/File:Wikibookslogo-sr-p-1.5x.png
2x png: https://commons.wikimedia.org/wiki/File:Wikibookslogo-sr-p-2x.png
(I already ran them through 'optipng -o7')