Project file maintenance, C# 7.2 (#108)

1. Updates all the NuGet packages (except CommandLineParser because they ruined their API)
2. Makes all projects use C# 7.2 explicitly. (not latest)
3. Use some C# 7.2 features like readonly structs and default literals.

Content.Tests/Content.Tests.csproj

@@ -16,6 +16,7 @@
     <FileAlignment>512</FileAlignment>
     <TestProjectType>UnitTest</TestProjectType>
     <OutputPath>..\bin\Content.Tests\</OutputPath>
+    <LangVersion>7.2</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DebugSymbols>true</DebugSymbols>
@@ -61,22 +62,18 @@
     <ErrorReport>prompt</ErrorReport>
   </PropertyGroup>
   <ItemGroup>
-    <Reference Include="nunit.framework, Version=3.10.1.0, Culture=neutral, PublicKeyToken=2638cd05610744eb, processorArchitecture=MSIL">
-      <HintPath>$(SolutionDir)packages\NUnit.3.10.1\lib\net45\nunit.framework.dll</HintPath>
-    </Reference>
-    <Reference Include="System.ValueTuple">
-      <HintPath>$(SolutionDir)packages\System.ValueTuple.4.4.0\lib\netstandard1.0\System.ValueTuple.dll</HintPath>
-    </Reference>
+    <PackageReference Include="NUnit" Version="3.10.1" />
+    <PackageReference Include="NUnit.ConsoleRunner" Version="3.9.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
+    <PackageReference Include="System.ValueTuple" Version="4.5.0" />
+  </ItemGroup>
+  <ItemGroup>
     <Reference Include="System" />
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Shared\Utility\ContentHelpers_Test.cs" />
   </ItemGroup>
-  <ItemGroup>
-    <None Include="packages.config" />
-  </ItemGroup>
-  <ItemGroup />
   <ItemGroup>
     <ProjectReference Include="..\Content.Client\Content.Client.csproj">
       <Project>{a2e5f175-78af-4ddd-8f97-e2d2552372ed}</Project>
@@ -127,4 +124,4 @@
   <Target Name="AfterBuild">
   </Target>
   -->
-</Project>
+</Project>