Fix #645 by calling Java Math.cbrt directly.

`Math.cbrt` was introduced in [Java 5](http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#cbrt%28double%29), so it is safe to use in the JRuby codebase.  Also, `cbrt` now throws a `TypeError` if the argument cannot be coerced to Float.

Related rubyspec change: https://github.com/rubyspec/rubyspec/pull/206