specify version of F# compiler in explicit way to workaround nuget bug, fixes #218

Author: adamsitnik

BenchmarkDotNet.Samples.FSharp.Core/project.json

@@ -23,7 +23,7 @@
   },
   "tools": {
     "dotnet-compile-fsc": {
-      "version": "1.0.0-*",
+      "version": "1.0.0-preview2-020000",
       "imports": [
         "dnxcore50",
         "portable-net45+win81",