Caching @ Travis is only available for private repositories

- see http://about.travis-ci.org/docs/user/caching/
exclude Maven meta-data from build artefacts

- otherwise pom.xml & pom.properties would be included under META-INF
- see http://eclipse.org/tycho/sitedocs/tycho-packaging-plugin/package-plugin-mojo.html
- see http://maven.apache.org/shared/maven-archiver/