Prepend Maven central in the pluginRepositories block

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.
Which artifact(s) do we need from ```imagej.thirdparty```?