feat: update stdlib mapping for go1.15

- drop stdlib for go1.13
- generate stdlib for go1.15
-  update CI configuration.

stdlib/go1_15_net_rpc_jsonrpc.go

@@ -0,0 +1,21 @@
+// Code generated by 'github.com/containous/yaegi/extract net/rpc/jsonrpc'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"net/rpc/jsonrpc"
+	"reflect"
+)
+
+func init() {
+	Symbols["net/rpc/jsonrpc"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"Dial":           reflect.ValueOf(jsonrpc.Dial),
+		"NewClient":      reflect.ValueOf(jsonrpc.NewClient),
+		"NewClientCodec": reflect.ValueOf(jsonrpc.NewClientCodec),
+		"NewServerCodec": reflect.ValueOf(jsonrpc.NewServerCodec),
+		"ServeConn":      reflect.ValueOf(jsonrpc.ServeConn),
+	}
+}