Fix robotstxt.org url

Looks like http://robotstxt.org does not resolve without www.
Remove unnecessary .gitkeeps

Given that there are files already present in these directories the `.gitkeep`s are unnecessary.
Merge pull request #2546 from karlfreeman/clean-up-gitkeeps

Remove unnecessary .gitkeeps
Merge pull request #2545 from karlfreeman/patch-1

Fix robotstxt.org url
use master of broccoli-sane-watcher to enable watchman support
force watchman on for now (until it is auto-detected by
broccoli-sane-watcher or sane)
allow GH deps
force watchman
auto-detection