Review retention policies/code for dl.hhvm.com contents

[fredemmott]

Related to #289

There are two likely places for this

• S3 bucket retention policies; this should be sufficient
• shell scripts to clean up the dl.hhvm.com XFS mount; I would expect deleting from s3 to lead to deletion here, e.g. via aws s3 sync, but this needs checking

Our current retention policies may be what we actually want - being able to run many-year-old versions easily is extremely useful sometimes - but we should check that we're not storing stuff that will definitely to be useful - for example, multi-year-old homebrew bottles are not going to be useful due to changes in the dependencies

That said, we shouldn't set 0% waste as a goal here, just clean up as much as is reasonable, trading off against developer time.