Fix travis builds

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Minimalistic subset of #872 to unblock our work.

[antonfirsov]

@iamcarbon in #872, everything else seems C# 8 related to me. Am I missing something?

[JimBobSquarePants]

Very odd. I thought #872 updated appveyor.yml to ensure the builds happen in VS2019.

I'm not sure how it builds now while targeting C#8.

[codecov]

Codecov Report

Merging #891 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
+ Coverage   88.97%   88.98%   +0.01%     
==========================================
  Files        1018     1018              
  Lines       44579    44537      -42     
  Branches     3234     3220      -14     
==========================================
- Hits        39663    39632      -31     
+ Misses       4195     4194       -1     
+ Partials      721      711      -10

Impacted Files	Coverage Δ
...ssing/Processors/Transforms/AutoOrientProcessor.cs	80.64% <0%> (-0.61%)	⬇️
...CC/DataReader/IccDataReader.MultiProcessElement.cs	90% <0%> (-0.48%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	93.25% <0%> (-0.36%)	⬇️
...ta/Profiles/ICC/DataReader/IccDataReader.Curves.cs	86.07% <0%> (-0.35%)	⬇️
src/ImageSharp/Formats/Bmp/BmpInfoHeader.cs	70.92% <0%> (-0.21%)	⬇️
...on/Implementation/Converters/HsvAndRgbConverter.cs	89.65% <0%> (-0.18%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	87.01% <0%> (-0.17%)	⬇️
src/ImageSharp/Processing/DetectEdgesExtensions.cs	96.42% <0%> (-0.13%)	⬇️
...iles/ICC/DataReader/IccDataReader.NonPrimitives.cs	93.65% <0%> (-0.1%)	⬇️
...files/ICC/DataReader/IccDataReader.TagDataEntry.cs	85.87% <0%> (-0.05%)	⬇️
... and 12 more

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[iamcarbon]

@JimBobSquarePants We only needed an SDK updated target C# 8.0 on AppVeyor. They updated the SDK in their standard image.