cmd/syncthing: Re-enable auto upgrade for dev builds (fixes #901)

As noted in the ticket I no longer agree that dev builds should not auto
upgrade. The main reason is that we give dev builds to users to test
specific fixes, and noone is happier by them being inadvertently stuck
on that version when a newer version including the fix is released.

For developers, it's first of all probably unlikely that development is
happening on a build that's older than release, and secondly STNOUPGRADE
can be set in the environment once and for all if it an issue.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3244

cmd/syncthing/main.go

@@ -786,10 +786,8 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 	if opts.AutoUpgradeIntervalH > 0 {
 		if noUpgrade {
 			l.Infof("No automatic upgrades; STNOUPGRADE environment variable defined.")
-		} else if IsRelease {
-			go autoUpgrade(cfg)
 		} else {
-			l.Infof("No automatic upgrades; %s is not a release version.", Version)
+			go autoUpgrade(cfg)
 		}
 	}