support JDK 11

[xuwei-k]

https://github.com/scalanlp/breeze/blob/09241acb5ca491c4b68c9b91d9e27bb7a25cdfc2/math/src/main/scala/breeze/linalg/functions/sum.scala#L11

Welcome to Scala 2.12.7 (Java HotSpot(TM) 64-Bit Server VM, Java 10.0.1).
Type in expressions for evaluation. Or try :help.

scala> import com.sun.istack.internal.Pool.Impl
import com.sun.istack.internal.Pool.Impl

Welcome to Scala 2.12.7 (Java HotSpot(TM) 64-Bit Server VM, Java 11).
Type in expressions for evaluation. Or try :help.

scala> import com.sun.istack.internal.Pool.Impl
<console>:11: error: object istack is not a member of package com.sun
       import com.sun.istack.internal.Pool.Impl
                      ^

[dlwh]

sorry about that