Add .net core 2.1 target (#83)

* added support for .net core 2.1

* remove wrongfully added project attribute

* run tests

Author: ajeckmans

src/SpecFlow.NetCore/SpecFlow.NetCore.csproj

@@ -1,9 +1,9 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <Description>Generate tests from SpecFlow feature files inside .NET Core projects.</Description>
     <Authors>stajs</Authors>
-    <TargetFrameworks>netcoreapp2.0;net46;net461</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.0;netcoreapp2.1;net46;net461</TargetFrameworks>
     <RuntimeIdentifiers>win7-x64;win7-x86;osx.10.10-x64;osx.10.11-x64;ubuntu.14.04-x64;ubuntu.16.04-x64;centos.7-x64;rhel.7.2-x64;debian.8-x64;fedora.23-x64;opensuse.13.2-x64</RuntimeIdentifiers>
     <RuntimeIdentifier Condition=" '$(TargetFramework)' == 'net46' OR '$(TargetFramework)' == 'net461' ">win7-x86</RuntimeIdentifier>
     <AssemblyName>dotnet-SpecFlow.NetCore</AssemblyName>