Fixed a compiler issue with portable.signed.

Author: niemyjski

src/Exceptionless.Portable.Signed/project.json

@@ -191,7 +191,7 @@
         "netstandard1.5": {
             "buildOptions": {
                 "debugType": "portable",
-                "define": [ "NO_SQL_CLIENT", "NETSTANDARD", "NETSTANDARD1_5" ]
+                "define": [ "SIGNED", "NO_SQL_CLIENT", "NETSTANDARD", "NETSTANDARD1_5" ]
             },
             "dependencies": {
                 "Microsoft.CSharp": "4.0.1-rc2-24027",