Fix logo URL (#4309)

app.json

@@ -2,7 +2,7 @@
   "name": "Mastodon",
   "description": "A GNU Social-compatible microblogging server",
   "repository": "https://github.com/tootsuite/mastodon",
-  "logo": "https://github.com/tootsuite/mastodon/raw/master/app/assets/images/logo.png",
+  "logo": "https://github.com/tootsuite/mastodon/raw/master/app/javascript/images/logo.svg",
   "env": {
     "HEROKU": {
       "description": "Leave this as true",

scalingo.json

@@ -2,7 +2,7 @@
   "name": "Mastodon",
   "description": "A GNU Social-compatible microblogging server",
   "repository": "https://github.com/tootsuite/mastodon",
-  "logo": "https://github.com/tootsuite/mastodon/raw/master/app/assets/images/logo.png",
+  "logo": "https://github.com/tootsuite/mastodon/raw/master/app/javascript/images/logo.svg",
   "env": {
     "LOCAL_DOMAIN": {
       "description": "The domain that your Mastodon instance will run on (this can be appname.scalingo.io or a custom domain)",