Fix case of all-zero response class

Happens if GBM gets a perfect prediction & more trees keep on coming.
Also, turn on GBMTest in the JUnits
Merge branch 'master' of github.com:0xdata/h2o
Turn off key-leaking GBMDomainTest

Real issue the new mapping Vecs are not being collected nicely.
Make cleaner versions of Vec.remove also.