Fixed build issue occurring with dotnet 2.1.500 (NuGet/Home#7591)

Author: jaaufauvre

.travis.yml

@@ -5,7 +5,7 @@ git:
 matrix:
   fast_finish: true
   include:
-  - dotnet: 2.1.500
+  - dotnet: 2.1.700
     mono: none
     env: 
       # Do not run analysis on PR (secure env variables like SONAR_TOKEN are not always available)
@@ -32,4 +32,4 @@ script:
   - dotnet test -c Release -l:"trx;LogFileName=tests.trx" -r:"bin/" /p:CollectCoverage=true /p:CoverletOutputFormat="opencover" /p:CoverletOutput="bin/coverage.xml"
   - dotnet publish -c Release
 after_success:
-  - test "$WITH_SONAR_ANALYSIS" = "true" && dotnet sonarscanner end /d:sonar.login="$SONAR_TOKEN" || echo 'Skipping sonarscanner end'
+  - test "$WITH_SONAR_ANALYSIS" = "true" && dotnet sonarscanner end /d:sonar.login="$SONAR_TOKEN" || echo 'Skipping sonarscanner end'