Fix build on go < 1.7

keepalive.go

@@ -0,0 +1,9 @@
+// +build go1.7
+
+package main
+
+import "runtime"
+
+func keepAlive(i interface{}) {
+	runtime.KeepAlive(i)
+}

keepalive_old.go

@@ -0,0 +1,7 @@
+// +build !go1.7
+
+package main
+
+func keepAlive(i interface{}) {
+	// Dummy function. Do nothing
+}

process.go

@@ -160,7 +160,7 @@ func calcCrop(width, height int, po processingOptions) (left, top int) {
 }
 
 func processImage(data []byte, imgtype imageType, po processingOptions) ([]byte, error) {
-	defer runtime.KeepAlive(data)
+	defer keepAlive(data)
 
 	err := C.int(0)