Failures on Mac (Mono master) and Max/Linux (Mono 4.05)

[dsyme]

I've enabled a build matrix in #476 but there are failures

https://travis-ci.org/fsharp/FSharp.Compiler.Service/jobs/94301490

@rneatherway If you get a chance to look that would be awesome. The Mac+mono master failure is in a recently adjusted part of the code

[rneatherway]

I think this is because the Mac version and old Mono are still populating the ChildProjectReferences correctly, so they now get duplicated. I can easily filter those out.