Fix for language-java 0.2.7

`language-java-0.2.7` introduced a breaking change in which the second argument of [`ArrayIndex`](https://github.com/vincenthz/language-java/blob/4c1880264f35f5ebd91313d259e2910824404db5/Language/Java/Syntax.hs#L380) is now an `[Exp]` instead of an `Exp`. This commit simply updates `IRTS.Java.ASTBuilding` to use the new `ArrayIndex`.