Updated NUnit to the latest version (3.6) to fix a test failing at AppVeyor.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

Author: ddobrev

build/InstallNugets.sh

@@ -1,5 +1,5 @@
 wget https://nuget.org/nuget.exe
 BUILD_DIR=$(dirname -- $0)
-mono nuget.exe install NUnit -Version 2.6.4 -OutputDirectory $BUILD_DIR/../deps
-mono nuget.exe install NUnit.Runners -Version 2.6.4 -OutputDirectory $BUILD_DIR/../deps
-cp $BUILD_DIR/../deps/NUnit.2.6.4/lib/nunit.framework.* $BUILD_DIR/../deps/NUnit
+mono nuget.exe install NUnit -Version 3.6.0 -OutputDirectory $BUILD_DIR/../deps
+mono nuget.exe install NUnit.Runners -Version 3.6.0 -OutputDirectory $BUILD_DIR/../deps
+cp $BUILD_DIR/../deps/NUnit.3.6.0/lib/nunit.framework.* $BUILD_DIR/../deps/NUnit