Merge pull request dotnet/corefx#9035 from AlexGhiondea/AppContextPackageVersion

Update the framework from netstandard1.5 to netstandard1.6 for AppContext

Commit migrated from https://github.com/dotnet/corefx/commit/63c23301cddb760ad0c2e529a7dac01daad129e4

src/libraries/System.AppContext/ref/project.json

@@ -3,7 +3,7 @@
     "System.Runtime": "4.0.0"
   },
   "frameworks": {
-    "netstandard1.5": {
+    "netstandard1.6": {
       "imports": [
         "dotnet5.6"
       ]

src/libraries/System.AppContext/src/project.json

 {
   "frameworks": {
-    "netstandard1.5": {
+    "netstandard1.6": {
       "dependencies": {
         "Microsoft.TargetingPack.Private.CoreCLR": "1.0.0-rc4-24201-00"
       },