EnvironmentExtensions still referenced in v5.0.0

[gitfool]

Looks like I missed something in #103 as I get an error when trying to use v5.0.0:

Compiling build script...
error CS0234: The type or namespace name 'EnvironmentExtensions' does not exist in the namespace 'Cake.Incubator' (are you missing an assembly reference?)
Error: Cake.Core.CakeException: Error(s) occurred when compiling build script:
error CS0234: The type or namespace name 'EnvironmentExtensions' does not exist in the namespace 'Cake.Incubator' (are you missing an assembly reference?)
   at Cake.Scripting.Roslyn.RoslynScriptSession.Execute(Script script)
   at Cake.Core.Scripting.ScriptRunner.Run(IScriptHost host, FilePath scriptPath, IDictionary`2 arguments)
   at Cake.Commands.BuildCommand.Execute(CakeOptions options)
   at Cake.CakeApplication.Run(CakeOptions options)
   at Cake.Program.Main()