Run system tests on CI (travis)

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some tests (for example resolve) can be run against oss.sonatype.org

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Or better use https://github.com/sonatype/docker-nexus (contains even professional edition)

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Travis supports running tests in Docker! https://docs.travis-ci.com/user/docker/

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creating a EL7 vm to try if nexus docker pro image cotains also a licence

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according to the dockerfile, it doesn't contain any license. https://github.com/sonatype/docker-nexus/blob/master/pro/Dockerfile