Prepend Maven central in the pluginRepositories block (rebased onto develop)


This is the same as gh-1236 but rebased onto develop.

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This should decrease the burden on our artifactory to resolve external dependencies in the Travis builds.

To review this PR, check the logs of the `maven` and `cppwrap` components of the Bio-Formats Travis build. In particular make sure that `artifacts.openmicroscopy.org` is only hit when necessary.