Use portable pdb.

Fixes issue #48

src/Renci.SshNet.NETCore/project.json

@@ -3,7 +3,7 @@
   "configurations": {
     "debug": {
       "buildOptions": {
-        "debugType": "full",
+        "debugType": "portable",
         "define": [ "DEBUG", "TRACE" ]
       }
     },