Tell search engines to ignore /pipermail/ mailing list archives

See issue #49, the mailman archives are still via e.g.

http://lists.open-bio.org/pipermail/biopython/
http://lists.open-bio.org/pipermail/biopython-dev/

and:

http://mailman.open-bio.org/pipermail/biopython/
http://mailman.open-bio.org/pipermail/biopython-dev/

However, with the move of biopython.org to GitHub Pages
we can no longer do this on our main domain.

robots.txt

@@ -15,6 +15,7 @@ Disallow: /DIST/docs/_api_163b/
 Disallow: /DIST/docs/_api_164/
 Disallow: /DIST/docs/_api_165/
 Disallow: /static/
+Disallow: /pipermail/
 Disallow: /info/
 Disallow: /w/index.php*
 Disallow: /wiki/Special:*