Get rid of desprecation warnings

Running "rake db:create && rake db:migrate" leads to
a deprecation warning for config option `config.whiny_nils`
and `active_record.auto_explain_threshold_in_seconds`
due to the fact that newer Rails and Ruby versions
dropped these options.

As graveio switch to Rails 4 and we are running
with Ruby >= 1.9.3 we can safely remove them.