Virtualised Build Agents (Docker) & Multi-version Testing

Tracking pull request for the work to support multi-version testing using virtualised build agents.

PHPCI will, by default, support Docker-based build agents. However, any server that matches the version requirements of a build will work.

### Tasks

- [ ] Create command to build a specific agent with Docker (`./console phpci:create-docker-agent`)
- [ ] Create some base Docker containers:
  - [ ] Ubuntu + PHP 5.5
  - [ ] Ubuntu + PHP 5.3
  - [ ] Ubuntu + HHVM
- [ ] Allow for parent / child builds
- [ ] Allow for a PHP version requirement on builds
- [ ] Add setting for PHP versions to test against
- [ ] Make child builds update the state of their parent build (if there is one)
- [ ] Display child builds in UI (if there are any)
- [ ] Only display parent builds in builds tables, project status table, etc.