[Chore] Bump swift snapshot testing to 1.7.2

[johnxnguyen]

What's in this PR?

This PR updates the swift snapshot testing dependency to 1.7.2 for Xcode 11.4 support.

[iwasrobbed-ks]

Think this will need an Xcode bump in .travis.yml

Module compiled with Swift 5.2.2 cannot be imported by the Swift 5.1 compiler: /Users/travis/build/iwasrobbed/Down/Carthage/Build/iOS/SnapshotTesting.framework/Modules/SnapshotTesting.swiftmodule/x86_64-apple-ios-simulator.swiftmodule

[codecov]

Codecov Report

Merging #216 into master will decrease coverage by 1.44%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   70.07%   68.63%   -1.45%     
==========================================
  Files          74       74              
  Lines        2042     2047       +5     
==========================================
- Hits         1431     1405      -26     
- Misses        611      642      +31     

Impacted Files	Coverage Δ
Tests/Styler/StylerTestSuite.swift	98.86% <100.00%> (+0.01%)	⬆️
...tyling/Attribute Collections/ColorCollection.swift	0.00% <0.00%> (-100.00%)	⬇️
...Styling/Attribute Collections/FontCollection.swift	0.00% <0.00%> (-90.91%)	⬇️
.../AST/Styling/Helpers/ListItemParagraphStyler.swift	67.50% <0.00%> (-7.50%)	⬇️
...yler/Helpers/NSAttributedString+HelpersTests.swift	0.00% <0.00%> (ø)
...rs/NSMutableAttributedString+AttributesTests.swift	0.00% <0.00%> (ø)
Source/AST/Visitors/AttributedStringVisitor.swift	91.52% <0.00%> (+0.07%)	⬆️

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