Warn and exit if boto version is too old

[jimbrowne]

Support for S3 encryption appeared in Boto 2.1.0; support for S3 reduced redundancy appeared in Boto 2.0. Sadly many Linux distributions still contain a pre-2.0 boto. It's better to proactively check that the available boto library is recent enough for boto-rsync to function.

[seedifferently]

@jimbrowne Thanks for the pull req. However, I'm not really sure why this is necessary seeing as the setup script requires boto 2.1.1 (https://github.com/seedifferently/boto_rsync/blob/master/setup.py#L7) and any of the "approved" methods for installing boto-rsync should go through that.

Are you expecting people to be installing/implementing this script in more unconventional ways?

[jimbrowne]

I noticed that this morning when re-packaging. Honestly I can't remember how I wound up with the script on the machine with an old boto version; could have been from just performing a git clone of the repo. I have a number of boto-related scripts that people tend to use directly from a git repo (c.f. https://github.com/jimbrowne/awstools) so it's a pattern I have baked into my reflexes. If your opinion is that only the "approved" installation methods should be supported I don't disagree with that.

[seedifferently]

Thanks, I will think on it.