Set DateFormatter.locale to en_US_POSIX (fixes #27)

[fabianfett]

No description provided.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   84.46%   84.49%   +0.02%     
==========================================
  Files          35       35              
  Lines        1667     1670       +3     
==========================================
+ Hits         1408     1411       +3     
  Misses        259      259

Impacted Files	Coverage Δ
Sources/LambdaEvents/S3.swift	89.28% <100%> (+0.39%)	⬆️
Sources/LambdaEvents/Cloudwatch.swift	88.88% <100%> (+0.42%)	⬆️
Sources/LambdaEvents/SNS.swift	66.23% <100%> (+0.44%)	⬆️

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