feat: update stdlib mapping for go1.15

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

stdlib/go1_15_container_ring.go

@@ -0,0 +1,20 @@
+// Code generated by 'github.com/containous/yaegi/extract container/ring'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"container/ring"
+	"reflect"
+)
+
+func init() {
+	Symbols["container/ring"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"New": reflect.ValueOf(ring.New),
+
+		// type definitions
+		"Ring": reflect.ValueOf((*ring.Ring)(nil)),
+	}
+}