use netcore sdk preview1

Author: enricosada

src/fsharp/FSharp.Compiler.Service.ProjectCracker.netcore/project.json

@@ -61,7 +61,7 @@
   },
   "tools": {
     "dotnet-compile-fsc": {
-      "version": "1.0.0-rc2-*",
+      "version": "1.0.0-preview1-*",
       "imports": [
         "dnxcore50",
         "portable-net45+win81",

src/fsharp/FSharp.Compiler.Service.netcore/project.json

@@ -290,7 +290,7 @@
   "tools": {
     "dotnet-fssrgen": "3.0.1",
     "dotnet-compile-fsc": {
-      "version": "1.0.0-rc2-*",
+      "version": "1.0.0-preview1-*",
       "imports": [
         "dnxcore50",
         "portable-net45+win81",