java 8 and scoobi: fix

[Alex Cozzi]

I was looking into the problem of running scoobi in Java 8 and I identified one of the problem as being the same as https://issues.jenkins-ci.org/browse/JENKINS-18537?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

that can be solved by updating xstream to the latest version:

at line 33 of project/dependencies.scala:

    "com.thoughtworks.xstream"          %  "xstream"                   % "1.4.8"            intransitive(),

after that I can run the test in local mode with Java 8 on my mac (with two failures in the test setting the log level, but I think those were happening anyhow)