fix project config

[unStatiK]

After this commit e9868f0 build process is broken.

Done Building Project "/usr/src/app/MahjongAI/MahjongAI.csproj" (default targets) -- FAILED.
Done Building Project "/usr/src/app/MahjongAI.sln" (Build target(s)) -- FAILED.

Build FAILED.

"/usr/src/app/MahjongAI.sln" (Build target) (1) ->
"/usr/src/app/MahjongAI/MahjongAI.csproj" (default target) (2) ->
(ResolveAssemblyReferences target) ->
  /usr/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(2218,5): warning MSB3245: Could not resolve this reference. Could not locate the assembly "Newto
nsoft.Json, Version=12.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed, processorArchitecture=MSIL". Check to make sure the assembly exists on disk. If this referen
ce is required by your code, you may get compilation errors. [/usr/src/app/MahjongAI/MahjongAI.csproj]

This commit fix project config for build.