Update the Travis-CI to use Catalina and enable macOS test case

[dreampiggy]

This enable the macOS test cases, previouslly travis has only support macOS 10.14...:)

[dreampiggy]

Travis does not still support macOS Catalina. Close currently :(
See: https://travis-ci.community/t/xcode-11-4-image-uses-macos-version-that-is-unsupported-by-that-version-of-xcode/8111

[dreampiggy]

News: https://changelog.travis-ci.com/xcode-11-4-1-is-now-available-149423

[codecov]

Codecov Report

Merging #98 into master will increase coverage by 2.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   71.58%   74.13%   +2.55%     
==========================================
  Files           9        9              
  Lines         834      901      +67     
==========================================
+ Hits          597      668      +71     
+ Misses        237      233       -4     

Flag	Coverage Δ
#ios	72.01% <ø> (+0.43%)	⬆️
#macos	75.42% <ø> (?)

Impacted Files	Coverage Δ
...eSwiftUI/Classes/Indicator/ProgressIndicator.swift	69.09% <0.00%> (-5.91%)	⬇️
...eSwiftUI/Classes/Indicator/ActivityIndicator.swift	93.10% <0.00%> (-0.23%)	⬇️
SDWebImageSwiftUI/Classes/ImageManager.swift	69.84% <0.00%> (+0.48%)	⬆️
SDWebImageSwiftUI/Classes/AnimatedImage.swift	74.44% <0.00%> (+0.84%)	⬆️
SDWebImageSwiftUI/Classes/ImageViewWrapper.swift	87.09% <0.00%> (+4.40%)	⬆️
SDWebImageSwiftUI/Classes/WebImage.swift	84.15% <0.00%> (+7.15%)	⬆️

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