Bump CUDA dep from 3,4,5 to 3,4,5,6 (#217)

According to [1] 6.0.0 should not be breaking

[1] https://github.com/JuliaGPU/CUDA.jl/releases/tag/v6.0.0

Author: thirtysixbananas

Project.toml

@@ -28,7 +28,7 @@ XGBoostTermExt = "Term"
 [compat]
 AbstractTrees = "0.4"
 CEnum = "0.4, 0.5"
-CUDA = "3, 4, 5"
+CUDA = "3, 4, 5, 6"
 JSON = "1.4.0"
 OrderedCollections = "1"
 SparseMatricesCSR = "0.6"