Add missing CFBundleVersion, bump version to 0.3.1

[MaxDesiatov]

Resolves #72

[codecov]

Codecov Report

Merging #74 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   75.62%   75.65%   +0.03%     
==========================================
  Files          31       31              
  Lines        1489     1491       +2     
==========================================
+ Hits         1126     1128       +2     
  Misses        363      363

Impacted Files	Coverage Δ
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	15.23% <ø> (ø)	⬆️
Sources/XMLCoder/Auxiliaries/Box/KeyedBox.swift	100% <100%> (ø)	⬆️

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