feat: update stdlib mapping for go1.15

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

stdlib/go1_15_os_signal.go

@@ -0,0 +1,21 @@
+// Code generated by 'github.com/containous/yaegi/extract os/signal'. DO NOT EDIT.
+
+// +build go1.15,!go1.16
+
+package stdlib
+
+import (
+	"os/signal"
+	"reflect"
+)
+
+func init() {
+	Symbols["os/signal"] = map[string]reflect.Value{
+		// function, constant and variable definitions
+		"Ignore":  reflect.ValueOf(signal.Ignore),
+		"Ignored": reflect.ValueOf(signal.Ignored),
+		"Notify":  reflect.ValueOf(signal.Notify),
+		"Reset":   reflect.ValueOf(signal.Reset),
+		"Stop":    reflect.ValueOf(signal.Stop),
+	}
+}