chore(travis): update osx_image to 9.3

[Liquidsoul]

[codecov[bot]]

Codecov Report

Merging #25 into master will increase coverage by 0.13%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   77.28%   77.42%   +0.13%     
==========================================
  Files           5        5              
  Lines         656      660       +4     
  Branches       84       84              
==========================================
+ Hits          507      511       +4     
  Misses        149      149

Impacted Files	Coverage Δ
...on/Extensions/AutoJSONSerializable.generated.swift	100% <ø> (ø)	:arrow_up:
.../Extensions/AutoJSONDeserializable.generated.swift	68.75% <100%> (+0.3%)	:arrow_up:

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