multi target (#56)

* multi target

* run on windows

* rem msbuild

Author: bitfaster

.github/workflows/dotnet-core.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: windows-latest
 
     steps:
     - uses: actions/checkout@v2

BitFaster.Caching/BitFaster.Caching.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>net472;net48;netstandard2.0;netcoreapp2.0;netcoreapp3.0;netcoreapp3.1</TargetFrameworks>
     <Authors>Alex Peck</Authors>
     <Company />
     <Product>BitFaster.Caching</Product>