Bump AWS SDK

Fixes https://github.com/docker/distribution/issues/3097

Signed-off-by: Yong Wen Chua <lawliet89@users.noreply.github.com>

vendor/github.com/aws/aws-sdk-go/internal/context/background_go1.5.go

@@ -0,0 +1,40 @@
+// +build !go1.7
+
+package context
+
+import "time"
+
+// An emptyCtx is a copy of the Go 1.7 context.emptyCtx type. This is copied to
+// provide a 1.6 and 1.5 safe version of context that is compatible with Go
+// 1.7's Context.
+//
+// An emptyCtx is never canceled, has no values, and has no deadline. It is not
+// struct{}, since vars of this type must have distinct addresses.
+type emptyCtx int
+
+func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (*emptyCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (*emptyCtx) Err() error {
+	return nil
+}
+
+func (*emptyCtx) Value(key interface{}) interface{} {
+	return nil
+}
+
+func (e *emptyCtx) String() string {
+	switch e {
+	case BackgroundCtx:
+		return "aws.BackgroundContext"
+	}
+	return "unknown empty Context"
+}
+
+// BackgroundCtx is the common base context.
+var BackgroundCtx = new(emptyCtx)