Serve robots.txt from /

This change allows `robots.txt` to be served from the root URL, i.e.
https://drone.io/robots.txt, which is where [web crawlers will look for
it][1]. Previously this file was only served from `/static/robots.txt`.

[1]: http://www.robotstxt.org/robotstxt.html

server/app/robots.txt

@@ -1 +1 @@
-User-agent: *
+User-agent: *

server/main.go

@@ -137,6 +137,7 @@ func main() {
 	// create handler for static resources
 	assets := rice.MustFindBox("app").HTTPBox()
 	assetserve := http.FileServer(rice.MustFindBox("app").HTTPBox())
+	http.Handle("/robots.txt", assetserve)
 	http.Handle("/static/", http.StripPrefix("/static", assetserve))
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		w.Write(assets.MustBytes("index.html"))