Remove IS NULL check in immortal model queries

Remove the IS NULL comparision is a decision to improve the perfomance
for all queries in immortal models
Remove specs that uses the IS NULL and update test schema
Add warn for deleted columns that accepts null values

  The warn will inform the users after update Immortal gem, the NULL
  value will not be valid as false. This decision was taken to improve
  the performance in queries for immortal models.
Add specs for warn in deleted column when is nullable
Merge pull request #12 from teambox/remove-null-check-on-sql

Remove null check on sql