try to fix build on travis

Author: AndrewKostousov

GrEmit.Tests/GrEmit.Tests.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <IsPackable>false</IsPackable>
-    <TargetFrameworks>net45;netcoreapp2.0;netcoreapp2.1;netcoreapp2.2</TargetFrameworks>
+    <TargetFrameworks>net45;netcoreapp2.0;netcoreapp2.1;netcoreapp2.2;netcoreapp3.1</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>

test-by-travis.sh

@@ -6,8 +6,8 @@ if [ ${DOTNETCORE} -eq 1 ]
 then
   dotnet restore ./GrEmit.sln --verbosity m
   dotnet build --configuration Release --framework netstandard2.0 ./GrEmit/GrEmit.csproj
-  dotnet build --configuration Release --framework netcoreapp2.2 ./GrEmit.Tests/GrEmit.Tests.csproj
-  dotnet test --no-build --configuration Release --framework netcoreapp2.2 ./GrEmit.Tests/GrEmit.Tests.csproj
+  dotnet build --configuration Release --framework netcoreapp3.1 ./GrEmit.Tests/GrEmit.Tests.csproj
+  dotnet test --no-build --configuration Release --framework netcoreapp3.1 ./GrEmit.Tests/GrEmit.Tests.csproj
 else
   nuget install NUnit.ConsoleRunner -Version 3.9.0 -OutputDirectory testrunner
   msbuild /t:Restore ./GrEmit.sln