Deprecation warnings when building against gazebo6

**Original report ([archived issue](https://osrf-migration.github.io/drcsim-gh-pages/#!/osrf/drcsim/issues/496)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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https://gist.github.com/scpeters/b182cc99aa374470ce4f
