Fix for onSize not rendering children (Issue: #136)

[hdwatts]

Previously setState was not being called if the 'strategy' was a callback, causing the child components to not get rendered.

This fixes issue #136 for me

[codecov]

Codecov Report

Merging #138 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #138   +/-   ##
=======================================
  Coverage   88.37%   88.37%           
=======================================
  Files           3        3           
  Lines          86       86           
  Branches       23       23           
=======================================
  Hits           76       76           
  Misses          8        8           
  Partials        2        2

Impacted Files	Coverage Δ
src/sizeMe.js	95% <100%> (ø)	⬆️

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[ctrlplusb]

Legend, thanks 👍