Run AppVeyor build using VS 2015 image
Run AppVeyor build using VS 2015 image

closes https://github.com/Microsoft/visualfsharp/pull/624

syeerzy/visualfsharp,Jand42/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,vasily-kirichenko/FSharp.Compiler.Service,syeerzy/visualfsharp,Heather/fsharp,dsyme/fsharp,forki/FSharp.Compiler.Service,matthid/FSharp.Compiler.Service,cloudRoutine/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,Jand42/FSharp.Compiler.Service,rneatherway/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,dsyme/fsharp,matthid/FSharp.Compiler.Service,syeerzy/visualfsharp,alfonsogarciacaro/FSharp.Compiler.Service,Heather/fsharp,brettfo/visualfsharp,In2TheWild/fsharp,sergey-tihon/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,dsyme/FSharp.Compiler.Service,dsyme/fsharp,In2TheWild/fsharp,dsyme/FSharp.Compiler.Service,syeerzy/visualfsharp,brettfo/visualfsharp,dsyme/fsharp,ncave/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,sergey-tihon/FSharp.Compiler.Service,vasily-kirichenko/FSharp.Compiler.Service,wk-j/fsharp,forki/FSharp.Compiler.Service,wk-j/fsharp,dsyme/FSharp.Compiler.Service,brettfo/visualfsharp,BillHally/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,dsyme/fsharp,syeerzy/visualfsharp,Jand42/FSharp.Compiler.Service,fsharp/fsharp,cloudRoutine/FSharp.Compiler.Service,forki/FSharp.Compiler.Service,fsharp/fsharp,vasily-kirichenko/FSharp.Compiler.Service,brettfo/visualfsharp,ncave/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,brettfo/visualfsharp,brettfo/visualfsharp,dsyme/fsharp,sergey-tihon/FSharp.Compiler.Service,dsyme/fsharp,syeerzy/visualfsharp,cloudRoutine/FSharp.Compiler.Service,matthid/FSharp.Compiler.Service,BillHally/FSharp.Compiler.Service,fsharp/fsharp,rneatherway/FSharp.Compiler.Service