Add allowedVersions property to packages.config

This makes the native binaries package dependency be locked to the specific
version listed in the file, preventing an accidental upgrade to a any newer
incompatible version that might be published.

This dependency is then used when the LibGit2Sharp NuGet package is built.

Author: bording

LibGit2Sharp/packages.config

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
-  <package id="Brandon.Test.NativeBinaries" version="0.0.5" targetFramework="net40" />
+  <package id="Brandon.Test.NativeBinaries" version="0.0.5" allowedVersions="[0.0.5]" targetFramework="net40" />
 </packages>