Enhancement: Run phpmd on Travis

This PR

* [x] adds `phpmd/phpmd` as a development dependency
* [x] updates `.travis.yml` to run `phpmd`
* [x] fixes fixable warnings reported by `phpmd`
* [x] suppresses unfixable warnings reported by `phpmd`

Related to and blocked by https://github.com/zendframework/modules.zendframework.com/pull/211.