Add log removal data

This is required for the new log removal feature. There is an existing PR (https://github.com/travis-ci/travis-core/pull/353), but it will most likely be reworked.

Ideally, we should be using [Sequel migrations](http://sequel.jeremyevans.net/rdoc/files/doc/migration_rdoc.html) to manage migrations here, but I get the following exception when I try to dump the current schema:

```
$ bx sequel -d postgres://localhost/travis_development
Error: Sequel::AdapterNotFound: LoadError: no such file to load -- pgorg/jruby/RubyKernel.java:1082:in `require'
```

So, for the time being, manage stuff with a very kludgy db alteration.