Behavior of Jruby Math.cbrt(number) does not match MRI

**jruby**
$ jruby -v

```
jruby 1.7.3 (1.9.3p385) 2013-02-21 dac429b on Java HotSpot(TM) 64-Bit Server VM 1.7.0_17-b02 [darwin-x86_64]
```

$ jruby -e "puts Math.cbrt(64)"

```
3.9999999999999996
```

**ruby**
$ ruby -v

```
ruby 1.9.3p385 (2013-02-06 revision 39114) [x86_64-darwin12.3.0]
```

$ ruby -e "puts Math.cbrt(64)"

```
4.0
```

**Math.sqrt behaves the same way as MRI**
