Serve robots.txt correctly in sandbox env.

I'm sick of getting the error emails.

sandbox/deploy/nginx/qa.conf

 	location /media/ {
 		root /var/www/oscar/builds/qa/sandbox/assets;
 	}
+    location =/robots.txt {
+		root /var/www/oscar/builds/sandbox/sandbox/public;
+	}
 	location /static/ {
 		root /var/www/oscar/builds/qa/sandbox/public;
 	}

sandbox/deploy/nginx/sandbox.conf

 	location /media/ {
 		root /var/www/oscar/builds/sandbox/sandbox/assets;
 	}
+    location =/robots.txt {
+		root /var/www/oscar/builds/sandbox/sandbox/public;
+	}
 	location /static/ {
 		root /var/www/oscar/builds/sandbox/sandbox/public;
 	}