Migrate from alexellis org to openfaas

Note, not all `alexellis/github` references should be changed, there are
a number of repos which are not part of the openfaas org, this commit
excludes those.

Signed-off-by: John McCabe <john@johnmccabe.net>

sample-functions/ApiKeyProtected/handler.go

@@ -7,7 +7,7 @@ import (
 	"net/http"
 	"os"
 
-	"github.com/alexellis/faas/watchdog/types"
+	"github.com/openfaas/faas/watchdog/types"
 )
 
 func handle(header http.Header, body []byte) {