make isFFWithBuggyScrollbar return false if UA does not contain OS X Firefox

Size diff between current branch and master
       23K      (+46B)  jquery.nanoscroller.js
      8.7K       (+7B)  jquery.nanoscroller.min.js
      2.9K       (+5B)  jquery.nanoscroller.min.js.gz
Add back a line that was accidentally deleted
Make sure that scroll values are updated before setting them on scroll

Fixes #196