[Chore] Use SPM to manage snapshot testing dependency

[johnxnguyen]

What's in this PR?

Changes

• Use SPM to manage the snapshot testing framework
• Remove Carthage files
• Re-record snapshots with iPhone 12 simulator
• Update travis config to use Xcode 12.2

[codecov-io]

Codecov Report

Merging #247 (00e70f4) into master (21d522d) will decrease coverage by 35.56%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master     #247       +/-   ##
===========================================
- Coverage   68.73%   33.17%   -35.57%     
===========================================
  Files          75       75               
  Lines        2050     2062       +12     
===========================================
- Hits         1409      684      -725     
- Misses        641     1378      +737     

Impacted Files	Coverage Δ
Tests/Styler/BlockQuoteStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/CodeBlockStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/DownDebugLayoutManagerTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/HeadingStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/InlineStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/LinkStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/ListItemStyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Tests/Styler/StylerTestSuite.swift	0.00% <0.00%> (-98.87%)	⬇️
Tests/Styler/ThematicBreakSyleTests.swift	0.00% <ø> (-100.00%)	⬇️
Source/AST/Nodes/Code.swift	0.00% <0.00%> (-100.00%)	⬇️
... and 56 more

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[johnxnguyen]

Codecov appears to be borked, will need to look into this.