Spark 1.5 comes with a new very efficient serializer that uses code generation. It is twice as fast as kryo. When using mahout, we have to set KryoSerializer because some classes aren't serializable otherwise.
I suggest to declare Math classes as "implements Serializable" where needed. For instance, to use coocurence package in spark 1.5, we had to modify AbstractMatrix, AbstractVector, DenseVector and SparseRowMatrix to make it work without Kryo.