github-commits: Use res.send to respond to requests

Instead of leaving folks hanging (which res.end without arguments
seems to do), use the higher-level res.send to send '200 OK' with the
default body [1].  With this commit, GitHub will know that the hook is
working, and not report 'Service Timeout' in the hook's details page.

There are a number of other 'res.end' calls in other scripts,
including a:

  res.end ""

call in github-pull-request-notifier.coffee (which seems to work), but
I think the higher-level interface and explicit clarity of:

  res.send 200

is more appropriate.

I didn't touch the 'res.end' calls in any other scripts, because the
pull-request notifier seems to be working as is, and I haven't enabled
the other res.end-using scripts for testing.

Fixes #1454.

[1]: http://expressjs.com/3x/api.html#res.send