Remove Private from System.Runtime reference (#258)

Author: josesimoes

source/TestFramework/nanoFramework.TestFramework.nfproj

@@ -59,7 +59,6 @@
     </Reference>
     <Reference Include="nanoFramework.System.Runtime, Version=1.0.27.0, Culture=neutral, PublicKeyToken=c07d481e9758c731">
       <HintPath>..\..\packages\nanoFramework.System.Runtime.1.0.27\lib\nanoFramework.System.Runtime.dll</HintPath>
-      <Private>True</Private>
     </Reference>
   </ItemGroup>
   <ItemGroup>