Microsoft released .NET 4.5.1 a while back, bringing a bunch of performance improvements and new features to the framework. Check out the announcement for the details.
Over the past few weeks we have updated our build infrastructure and application servers to support this release. We're happy to report that AppHarbor now supports building, testing and running applications targeting the .NET 4.5.1 framework, as well as solutions created with Visual Studio 2013 and ASP.NET MVC 5 applications.
There are no known issues related to this release. If you encounter problems, please refer to the usual support channels and we'll help you out.