Prepare version 3.1.0 for release - missing part.

Author: gdziadkiewicz

src/Directory.Build.props

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
   <PropertyGroup>
-    <VersionPrefix>3.0.0</VersionPrefix>
+    <VersionPrefix>3.1.0</VersionPrefix>
     <Owners>Steffen Forkmann and others</Owners>
     <Authors>Steffen Forkmann and others</Authors>
     <PackageProjectUrl>https://github.com/fsprojects/FSharpx.Extras</PackageProjectUrl>