feat: update stdlib mapping for go1.15

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

stdlib/go1_15_image_gif.go

@@ -0,0 +1,30 @@
+// Code generated by 'github.com/containous/yaegi/extract image/gif'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"go/constant"
+	"go/token"
+	"image/gif"
+	"reflect"
+)
+
+func init() {
+	Symbols["image/gif"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"Decode":             reflect.ValueOf(gif.Decode),
+		"DecodeAll":          reflect.ValueOf(gif.DecodeAll),
+		"DecodeConfig":       reflect.ValueOf(gif.DecodeConfig),
+		"DisposalBackground": reflect.ValueOf(constant.MakeFromLiteral("2", token.INT, 0)),
+		"DisposalNone":       reflect.ValueOf(constant.MakeFromLiteral("1", token.INT, 0)),
+		"DisposalPrevious":   reflect.ValueOf(constant.MakeFromLiteral("3", token.INT, 0)),
+		"Encode":             reflect.ValueOf(gif.Encode),
+		"EncodeAll":          reflect.ValueOf(gif.EncodeAll),
+
+		// type definitions
+		"GIF":     reflect.ValueOf((*gif.GIF)(nil)),
+		"Options": reflect.ValueOf((*gif.Options)(nil)),
+	}
+}