Clarify robots.txt for files.opentreeoflife.org

Regarding #348, this should prevent attempts to index large .tgz files.

N.B. We can't exclude *.tgz files per se, only block the paths where
they are located. This will block the explanation pages at /trees and
/ott, as well as other files on these paths. (If that's no good, we
should quarantine the .tgz files on a separate path and block it.)