Bring back compatibility with go1.4.2

- apa102 benchmark used image.CMYK, which was added in go 1.5. This has the side
  effect of making benchmark BenchmarkDrawSlowpath a bit faster.
- mpu9250 used type aliasing by accident, which was added in go1.9.
- periph-web used http.Server.Shutdown(), which was added in go1.8.

devices/apa102/apa102_test.go

@@ -1049,12 +1049,11 @@ func BenchmarkDrawRGBAColorfulPassThru(b *testing.B) {
 
 func BenchmarkDrawSlowpath(b *testing.B) {
 	// Should be an image type that doesn't have a fast path
-	img := image.NewCMYK(image.Rect(0, 0, 150, 1))
+	img := image.NewGray(image.Rect(0, 0, 150, 1))
 	for x := 0; x < img.Bounds().Dx(); x++ {
 		for y := 0; y < img.Bounds().Dy(); y++ {
 			pix := genColorfulPixel(x)
-			c := color.CMYK{C: pix[0], M: pix[1], Y: pix[2], K: 0xFF}
-			img.Set(x, y, c)
+			img.Set(x, y, color.Gray{pix[0]})
 		}
 	}
 	o := DefaultOpts

experimental/devices/mpu9250/spi.go

@@ -10,7 +10,7 @@ import (
 )
 
 // DebugF the debug function type.
-type DebugF = func(string, ...interface{})
+type DebugF func(string, ...interface{})
 
 // SpiTransport Encapsulates the SPI transport parameters.
 type SpiTransport struct {