fix(Dropdown): automatic upward logic in IE11

[mihai-dinculescu]

Replacing DOMRect.y with DOMRect.top because the latter seems to work in all browsers, including IE11.

[codecov-io]

Codecov Report

Merging #2885 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2885   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files         161      161           
  Lines        2733     2733           
=======================================
  Hits         2724     2724           
  Misses          9        9

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6d6f6eb...68d6890. Read the comment docs.

[levithomason]

Released in semantic-ui-react@0.81.2.