Fixing tags

This PR changes a way we handle branches and tags.

* tag is now saved in a `tag` column on both `Commit` and `Build`
* for commits with both tag and branch specified (this is specified in GitHub payload as `ref: 'refs/tags/tag-name` and `base_ref: 'refs/heads/branch-name`), we save both values
* a `tags` entry can be used in `.travis.yml` in order to filter builds by tag, for example: `tags: { only: '/deploy-/' }`

It's all put behind `:proper_tags` feature flag, which can be enabled for individual repositories.