Java svm uses faster `Math.pow` instead of `powi`

[Druid-of-Luhn]

Using the built-in Math.pow method instead of svm's custom powi. This makes the code faster.

I checked this by sampling svm prediction with jvisualvm. powi used to take about 8-10% of the CPU time (with Kernel.dot taking 66-73%). After changing it to Math.pow the time it takes is negligible.