speed: fix build for Go 1.4 and lower

Old Go versions miss cipher.NewGCMWithNonceSize, which causes:

  internal/speed/speed.go:95: undefined: cipher.NewGCMWithNonceSize

internal/speed/speed.go

@@ -1,3 +1,5 @@
+// +build go1.5
+
 // Package speed implements the "-speed" command-line option,
 // similar to "openssl speed".
 // It benchmarks the crypto algorithms and libraries used by

internal/speed/speed_go1.4.go

@@ -0,0 +1,11 @@
+// +build !go1.5
+
+package speed
+
+import (
+	"fmt"
+)
+
+func Run() {
+	fmt.Printf("Compile with Go 1.5 or higher to run the benchmarks\n")
+}

internal/speed/speed_test.go

@@ -1,3 +1,5 @@
+// +build go1.5
+
 package speed
 
 /*