Fixed assembly version to the correct value of 2.0.13 beta 1.

Internally the version numbers of the beta are coded with a vlue
starting with 1x.9x where 12.91 is beta1, 12.92 is beta2 to indicate it is close
to the final release 13.0. Incorrectly I used a wrong value.

src/Npgsql/AssemblyInfo.cs

@@ -24,7 +24,7 @@
 [assembly: AssemblyProductAttribute("Npgsql")]
 [assembly: AssemblyCopyrightAttribute("Copyright © 2002 - 2013 Npgsql Development Team")]
 [assembly: AssemblyTrademarkAttribute("")]
-[assembly: AssemblyVersionAttribute("2.0.13.91")]
-[assembly: AssemblyFileVersionAttribute("2.0.13.91")]
+[assembly: AssemblyVersionAttribute("2.0.12.91")]
+[assembly: AssemblyFileVersionAttribute("2.0.12.91")]
 [assembly: NeutralResourcesLanguageAttribute("en", UltimateResourceFallbackLocation.MainAssembly)]