Rails 4.2 Add `null: true` to avoid DEPRECATION WARNING

This pull request addresses following deprecation warning.
```ruby
.DEPRECATION WARNING: `timestamp` was called without specifying an option for `null`. In Rails 5.0, this behavior will change to `null: false`. You should manually specify `null: true` to prevent the behavior of your existing migrations from changing. (called from block (2 levels) in create_test_posts_table at /home/yahonda/git/oracle-enhanced/spec/active_record/connection_adapters/oracle_enhanced_schema_dump_spec.rb:25)
```