Save pull_request_title in Build

Simple change to save pull request title in the build model, to not need fetching a `Request` to get it.

Although I would just merge it right away, I have a mixed feeling about the direction and I would consider splitting Build into `Push` and `PullRequest` types, we could use [postgresql table inheritance](http://www.postgresql.org/docs/9.0/static/ddl-inherit.html) for this.