Add scala-xml for scala 2.11.

Xml is needed by ServerStateObserver...
Fix RoundRobinLB: if no server matches the query None is returned.

Previously it failed with a
    java.lang.ArithmeticException: / by zero
Only update server map if the state really changed.

The cluster state might have changed without any effect on our
server map, so that until now we might have logged "Updated
server map" even if it wasn't changed.
Update solr and curator to the latest releases (4.10.2, 2.7.0)

The timeout also had to be increased for CloudSolrServersIntegrationSpec.