Update .travis.yml for Xcode 9.3 beta
Update .travis.yml for Xcode 9.3 beta

According to the Travis docs they don't offer betas anymore, [but I've seen this been used about](https://github.com/IngmarStein/Monolingual/commit/e045774357e5e8b8f7204a35c258627e000e3da3#diff-354f30a63fb0907d4ad57269548329e3R4)
kellanburket/franz,kellanburket/franz