allow net45 compilation for abstractions so some common SDKs may take a dependency on it (#86)

Author: SergeyKanzhelev

src/OpenCensus.Abstractions/OpenCensus.Abstractions.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildProjectDirectory), 'OpenCensus.sln'))\build\Common.prod.props" />
   <PropertyGroup>
-    <TargetFrameworks>net46;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
     <TargetFrameworks Condition="$(OS) != 'Windows_NT'">netstandard2.0</TargetFrameworks>
     <Description>OpenCensus .NET API abstractions</Description>
     <IncludeSymbols>True</IncludeSymbols>