GBRT checks if ``loss`` is in self.supported_loss
renamed supported_loss to _SUPPORTED_LOSS (constants)
add test for deprecated warning
Merge pull request #2308 from pprett/gbrt-check-supported-loss

GBRT checks if ``loss`` is in self.supported_loss
Issue #2185: Fixed MinibatchKMeans bad center reallocation which caused duplicate centers
made _pick_unique_labels private, fixed misspellings, added unit test _pick_unique_labels
Fixed formatting issue