Prepend Maven central in the pluginRepositories block

This should decrease the burden on our artifactory to resolve external
dependencies in the Travis builds.
Prepend formats-common repositories with Maven Central (seralizer, xalan...)
Prepend autogen/pom.xml with Maven Central (velocity)
Prepend format-bsd/pom.xml repositories with Maven Central (perf4j)
Prepend formats-gpl/pom.xml repositories with unidata (netcdf)
Remove duplicate repository and prepend with ome.external (JWlz)
Prepend formats-bsd/pom.xml plugin repositories with Maven and LOCI
Add imagej third party repository to formats-gpl/pom.xml for jace
Merge pull request #1236 from sbesson/maven_pluginRepository

Prepend Maven central in the pluginRepositories block