Set defaults for JSONb postgres columns in Rails

Make sure to pass the migration a native Ruby hash as the default value. DO NOT pass it a string representation of an hash, thinking that it'll work (as valid JSON).

DO THIS

```ruby
t.jsonb :preferences, default: {}, null: false
```

NOT

```ruby
t.jsonb :preferences, default: '{}', null: false
```
It'll break in a maddeningly non-obvious way. Take my word for it. Also there is this
[relevant StackOverflow post](http://stackoverflow.com/questions/37986984/ruby-on-rails-jsonb-column-default-value) which saved my ass.

obie-fernandez
January 6, 2017