Snapshot-related updates + cap tasks

[EvanK]

Several improvements made to the backup script itself:

• Resolved some warnings regarding development versions and ssl cert checking
• Added --list, --retrieve and --force modes
• Revised mysqldump to match the output format of wp-cli's db export and our cap task's :db:backup
• Resolved string casting issue for retention periods

Also a couple provisioning changes:

• backup script (and any apt/pip dependencies) will be installed on all stages
• added pip install of certifi, for ssl cert updates
• the backup cron job is still installed only on production

As well, I've refactored some of the cap tasks:

• Gemfiles have been updated, with an additional gem dependancy
• resolved some broken stage-specific checks (was comparing symbols as strings)
• database importing and file rsyncing abstracted out into :import tasks respectively
• added snapshot-specific tasks (with docs!)
  • :snapshot:simulate easily simulates backup history for the past year, given a chosen config
  • :snapshot:force allows on-demand backup creation, outside of snapshot interval and without any retention checks
  • :snapshot:restore allows easily restoring existing snapshots to local or prod