Switch order of Mono search paths

[nosami]

When resolving assemblies using simpleResolution on Mono, the /4.5-api/ folder is used instead of the /4.5/ folder. This folder contains facade assemblies only.

✔ ~/src/fsharp/FSharp.Compiler.Service [master ↓·6|✚ 1…2⚑ 2]
15:01 $ fsharpi

F# Interactive for F# 4.1
Freely distributed under the Apache 2.0 Open Source License

For help type #help;;

> #r "System.Xml";;

--> Referenced '/Library/Frameworks/Mono.framework/Versions/4.8.0/lib/mono/4.5-api/System.Xml.dll' (file may be locked by F# Interactive process)

>
- Interrupt

Not sure if this PR has any knock on effects.

[dsyme]

Could you also submit to https://github.com/fsharp/fsharp please? Thanks