Support Mongoid 3.1.6

These changes can break support to other versions of mongoid.

Changes include:
- New way to skip setting updated_at
- Selector in atomic_selector uses strings, instead symbols (Fix for: undefined method `sub' for :migration_state:Symbol)
Changing rake task name to lazy_migrate
Make migration slower to ensure good website perf

Introduce batch_size
sleep every 100 records.
Missing Gemfile.lock changes
Comment to check the query performance