Travis: allow build failure for 2.9.3 on openjdk6

In the past days, this particular combination failed or errored because of
underlying system failure (JVM died, unable to create native threads, ...).
Hence, disabling it for now.
Revert "remove MetricSpace and BKTree"

This reverts commit 468e425c749dbe48349e288bbbc9d6159296f13a (#347).
deprecation warnings for `BKTree` and `MetricSpace`
work around a scalac warning wrt annotations and named arguments