travis: Turn off core dumps on OSX
travis: Turn off core dumps on OSX

I've seen these take up quite a bit of log space and I have the sneaking
suspicion that they're just making our test suite take longer (sometimes timing
out on 32-bit OSX now). In any case the backtraces haven't proven too useful,
unfortunately.

graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust