Use long-hand `Hash#slice` + `Hash#keys` instead of `Hash#except`

[al-the-x]

Since Hash#except is not in standard Ruby and Vagrant removed ~~active_support~~ i18n that provides the polyfill in Vagrant 2.2.7; even the polyfill was just Hash#slice minus the requested keys. Seemed simpler to inline than monkey-patch Hash...

Resolves mitchellh/vagrant-aws#566

[PxSonny]

I confirm it works good for me.

[vohi]

Thanks, needed to make a similar patch to my installation of the plugin on a macOS 10.14 host with vagrant 2.2.9