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
Merge pull request #770 from msteinert/robots-txt

Serve `robots.txt` from `/`