Update s3sync.gemspec

[eropple]

The patch-level semantic versioning isn't necessary here; AWS's API is stable through v2.

[clarete]

Hi @eropple! Thanks a lot for the patch, would you mind also pushing your new Gemfile.lock to this PR? I think that would fix this lil issue on travis:

(...)
$ bundle install --jobs=3 --retry=3 --deployment
You are trying to install in deployment mode after changing
your Gemfile. Run `bundle install` elsewhere and add the
updated Gemfile.lock to version control.
(...)