Add support for Swift 5

[pepicrft]

Short description 📝

Adds support for Swift 5

Implementation 👩‍💻👨‍💻

• Update .swift-version
• Replace SwiftShell with Shell
• Add swift_version to .podspec.

[codecov]

Codecov Report

Merging #397 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   78.06%   78.06%   +<.01%     
==========================================
  Files         140      140              
  Lines        5994     5996       +2     
==========================================
+ Hits         4679     4681       +2     
  Misses       1315     1315

Impacted Files	Coverage Δ
...s/xcodeprojTests/Objects/Files/PBXGroupTests.swift	88.67% <100%> (-0.42%)	⬇️
...ests/Objects/Project/PBXProjIntegrationTests.swift	100% <100%> (ø)	⬆️

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