fix(Dropdown): Fix SSR by removing direct reference to Element in prop types

[kylealwyn]

Gatsby breaks when building:

11:24:58 AM:   2842 |   transitionDuration: PropTypes.number,
11:24:58 AM:   2843 |   transitionTimingFunction: PropTypes.string,
11:24:58 AM: > 2844 |   portalNode: PropTypes.instanceOf(Element)
11:24:58 AM:        |                        ^
11:24:58 AM:   2845 | };
11:24:58 AM:   2846 | Dropdown.defaultProps = {
11:24:58 AM:   2847 |   placement: 'bottom-start',
11:24:58 AM: 
11:24:58 AM:   WebpackError: ReferenceError: Element is not defined

[codecov]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   68.36%   68.36%           
=======================================
  Files          22       22           
  Lines         433      433           
  Branches       92       92           
=======================================
  Hits          296      296           
  Misses        107      107           
  Partials       30       30

Impacted Files	Coverage Δ
src/Dropdown/Dropdown.js	92.85% <ø> (ø)	⬆️

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