Support newer versions of s3cmd (1.5.0 beta1)

[dimroc]

There's been many improvements since the 1.0.0 release three years ago.

[motns]

The reason why we're "stuck" on 1.0.0 is because we're using the package manager to install s3cmd for us, and it seems that 1.0.0 is the last "stable" release available there.
The alternative is to install manually from the source on GitHub - from what I remember the last time I looked at it, it can be a bit of a pain because of all the dependencies, but it's certainly possible.

Also, as per the note published here:
http://s3tools.org/repositories
there seems to be a way to get the pre-release package installed, but it doesn't look like that's available for Ubuntu, which is what we support primarily.

I agree that we should support more recent versions, so I'll look into installing from source when I have a moment.