Fix mouse wheel scroll-up not working

[pszoldan]

This is a bugfix commit. Scrolling with the mouse wheel only works downwards, but not upwards in the open selectric panel. This is because the condition to prevent scrolling up when we're already at the top takes the position from the wrong element and it always thinks it's scrolled all the way up, preventing any further scroll upwards. This commit fixes it by taking the scroll position from the correct element, the parent.

[codecov-io]

Codecov Report

Merging #210 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #210   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         391    391           
=====================================
  Hits          391    391

Impacted Files	Coverage Δ
src/jquery.selectric.js	100% <100%> (ø)	:arrow_up:

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[gearsdigital]

Thank you very much for you PR. I'll verify and test this in the next days.

[dm-AS]

The fix works for me. Thanks @pszoldan