Revert "This doesn't build for me locally, but apparently 5.2 should work; testing for CI server"

This reverts commit 4a70b621.

Dapper.StrongName/project.json

@@ -42,7 +42,7 @@
         "define": [ "ASYNC" ]
       }
     },
-    "dotnet5.2": {
+    "dotnet5.4": {
       "compilationOptions": {
         "define": [ "ASYNC", "COREFX" ]
       },

Dapper/project.json

@@ -40,7 +40,7 @@
         "define": [ "ASYNC" ]
       }
     },
-    "dotnet5.2": {
+    "dotnet5.4": {
       "compilationOptions": {
         "define": [ "ASYNC", "COREFX" ]
       },