go/build: add go1.3 release tag

Fixes #7918

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/91980043

src/pkg/go/build/build.go

@@ -292,10 +292,10 @@ func defaultContext() Context {
 	// say "+build go1.x", and code that should only be built before Go 1.x
 	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
 	//
-	// When we reach Go 1.3 the line will read
-	//	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3"}
+	// When we reach Go 1.4 the line will read
+	//	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4"}
 	// and so on.
-	c.ReleaseTags = []string{"go1.1", "go1.2"}
+	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3"}
 
 	switch os.Getenv("CGO_ENABLED") {
 	case "1":