Fix usage of Cake.Recipe on Travis

[gep13]

When running Cake.Recipe on Travis, the following error occurs:

https://travis-ci.org/cake-contrib/Cake.Plist/jobs/209156414#L1435

Compiling build script...
Error: Cake.Core.CakeException: An error occurred while executing build script.
  at Cake.Scripting.Mono.MonoScriptSession.Execute (Cake.Core.Scripting.Script script) [0x0007f] in <077b4364678d402f9df8bb48e9f0a543>:0 
  at Cake.Core.Scripting.ScriptRunner.Run (Cake.Core.Scripting.IScriptHost host, Cake.Core.IO.FilePath scriptPath, System.Collections.Generic.IDictionary`2[TKey,TValue] arguments) [0x00314] in <78602f51bc554886aeaf7e173d0ce2e7>:0 
  at Cake.Commands.BuildCommand.Execute (Cake.CakeOptions options) [0x0002d] in <077b4364678d402f9df8bb48e9f0a543>:0 
  at Cake.CakeApplication.Run (Cake.CakeOptions options) [0x0001d] in <077b4364678d402f9df8bb48e9f0a543>:0 
  at Cake.Program.Main () [0x000dc] in <077b4364678d402f9df8bb48e9f0a543>:0

I have been able to reproduce this error by compiling and running Cake on an Ubuntu VM, using JetBrains Rider. The issue comes down to the Cake.AppVeyor addin.

Now, some of you might be thinking, why are you using the Cake.AppVeyor addin when you are running on Travis, and this is due to the fact that you can't conditionally reference an addin, so it has to be included.

While debugging, it is the following method in the Cake.AppVeyor addin that is causing the problem:

Which reports the following error message:

System.Nullable [System.Int32] is not a supported constant type

The suspicion is that something in the Mono Compiler for Cake is either confused, or does not support what this method is trying to do.

[devlead]

This is the generated code causing the issue

public Cake.AppVeyor.AppVeyorProjectHistory AppVeyorProjectHistory(Cake.AppVeyor.AppVeyorSettings settings, System.String accountName, System.String projectSlug, System.Int32 recordsPerPage, System.Nullable<System.Int32> startBuildId = null, System.String branch = null)
{
    return Cake.AppVeyor.AppVeyorAliases.AppVeyorProjectHistory(Context, settings, accountName, projectSlug, recordsPerPage, startBuildId, branch);
}

[devlead]

This is the method being invoked:
http://cakebuild.net/api/Cake.AppVeyor/AppVeyorAliases/2C3663E7

public static AppVeyorProjectHistory AppVeyorProjectHistory(this ICakeContext context, AppVeyorSettings settings, string accountName, string projectSlug, int recordsPerPage, int? startBuildId = null, string branch = null)