increase s3_url_ttl from 15m to 8h

tired of endless CHEF-3045 complaints on the mailing list.

403s on long chef runs are not delightful.

any attacker than can issue a replay attack inside of an 8 hour window
can also issue a replay attack within a 15 minute window, so having this
timeout be shorter does not prevent any attacks (15 minutes is already
an eternity to any kind of automated attack tool).
Merge pull request #63 from opscode/lcg/increase-s3-ttl

increase s3_url_ttl from 15m to 8h