Update .travis.yml to test release and nightlies

Looks like we weren't testing juliarelease on Travis. Ah, the hazards of packages established at the dawn of time before we had all this lovely technology :smile:.

This should help things going forward. It took me about 6 attempts to get this simple change to the `.travis.yml` file right. One issue that cropped up towards the end is that the new JLDArchives tests won't work on julia 0.2, because UnitRange (one of the types saved) didn't exist then. So it may be that changes in julia itself are the biggest threat to the long-term stability of *.jld files. (One can use `readsafely`, of course.)