.Net core versions

[Nikolay Izhikov]

Hello. Igniters.

I working on some issue in .Net platforms and found that different project 
files for .Net contains different platform versions.
Can someone explain why?

1. Apache.Ignite.Core.csproj [1]

```
<PropertyGroup>
...
  <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
...
</PropertyGroup>
```

2. Apache.Ignite.Core.DotNetCore.csproj

```
<Project Sdk="Microsoft.NET.Sdk»>
…
    <TargetFramework>netcoreapp2.0</TargetFramework>
…
</Project>
```