Implement smooth gradual upgrades.

In this mode, einhorn will cull old workers as soon as a replacement
modern worker has acked, rather than waiting until all modern workers
are up and culling old workers all at once.
Make smooth upgrades the default.

The old upgrade behavior is now known as upgrade_fleet to einhornsh.
Add time_server code used for testing upgrades.
Add pry to development dependencies.
Launch <= NCPU workers at once in smooth upgrade.
Add Einhorn::VERSION to ehlo message.
Dedup gemspec dependencies.

Introduced by 07e0a948 and 828de765.
Version 0.5.5
Merge pull request #32 from stripe/ab-smooth-upgrades

Upgrade smoothly by default