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
Add Dropbox plugin
Merge pull request #770 from msteinert/robots-txt

Serve `robots.txt` from `/`
Merge pull request #768 from mopemope/dropbox

Add Dropbox plugin
Automatically detect ENV variables
Merge pull request #765 from Bugagazavr/detect_envs

Automatically detect ENV variables
Add an option to disable recursive clone

This change adds the following configuration item to .drone.yml:

    git:
      recursive: false

If the item is left out then the default is `true`, i.e. perform a
recursive clone.