Fix deprecation warning logic

- `ENV['PB_IGNORE_DEPRECATIONS']` now only controls field deprecations,
  as indicated in the documentation. Protobuf internal deprecations
  are always printed.
- Field deprecations now only print when the field is actually
  deprecated, and provide a more useful message.
Refresh RuboCop config
Restore Rails 3.2 compatibility

Closes #239.
Merge pull request #241 from tamird/rails-3.2-compat

Restore Rails 3.2 compatibility