Updated list of valid architectures

It's an attempt to help on #286 to fix failing builds on Travis CI: [15](https://travis-ci.org/allending/Kiwi/builds/7445343), [16](https://travis-ci.org/allending/Kiwi/builds/7445381), [17](https://travis-ci.org/allending/Kiwi/builds/7446176), [18](https://travis-ci.org/allending/Kiwi/builds/7446268).

But seems like right now builds are pass... [21](https://travis-ci.org/allending/Kiwi/builds/7457403)

Not sure why it was fail before...

But in any case changes in this commit seems reasonable to me.