Update Travis to build xcdiff with Xcode 11.5

[kwridan]

Describe your changes

• Updated Travis to build xcdiff using Xcode 11.5
• Updated the script used by Travis to leverage && between commands to ensure failures are propagated incase one of the commands fails
• Updated one of the fixtures that was failing CI (due to merge order)

Testing performed
Ran make test

Additional context
This should also switch the macOS version travis uses to Catalina.

[codecov]

Codecov Report

Merging #60 into master will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   95.82%   96.12%   +0.29%     
==========================================
  Files          43       43              
  Lines        1486     1598     +112     
==========================================
+ Hits         1424     1536     +112     
  Misses         62       62              

Impacted Files	Coverage Δ
Sources/XCDiffCore/ProjectComparator.swift	100.00% <ø> (ø)
Sources/XCDiffCommand/CommandRunner.swift	95.49% <100.00%> (+0.03%)	⬆️
...ultRenderer/TextProjectCompareResultRenderer.swift	100.00% <100.00%> (ø)
...Core/ResultRenderer/Renderer/ConsoleRenderer.swift	100.00% <100.00%> (ø)
...iffCore/ResultRenderer/Renderer/HTMLRenderer.swift	100.00% <100.00%> (ø)
...ore/ResultRenderer/Renderer/MarkdownRenderer.swift	100.00% <100.00%> (ø)
...fCore/ResultRenderer/UniversalResultRenderer.swift	100.00% <100.00%> (ø)
...rces/XCDiffCore/Comparator/HeadersComparator.swift	100.00% <0.00%> (ø)
...rces/XCDiffCore/Comparator/SourcesComparator.swift	100.00% <0.00%> (ø)
... and 13 more

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[marciniwanicki]

LGTM