Migration to fix tweets.id type to big int for new databases

Currently closing the project and running the migrations will fail because the migrations tries to alter tables who were not created before (by the migrations)...  the only way to get the data base up quickly now is to run 

```bash
rake db:reset
```
which will use the content of db/schema.rb file to create the schema... the missing part is that the tweets.id will be a regular integer with not enough value to hold the tweets id  [politwoops-tweet-collector issues #4](https://github.com/sunlightlabs/politwoops-tweet-collector/issues/4)

this pull requests adds a migration to change the tweets.id type to unsigned bigint so the system gets to work well