Update travis to always use the latest release

[garyb]

I'm not sure where exactly this should be applied, so if merging here is no good just close it and take the updated .travis.yml.

[joneshf]

I think in the absence of a real dependency manager like npm or bower I'd rather if we had to specifically state the version we're using. The latest release doesn't always work with every library, and I'd rather if we only used versions we know can compile this library. We can make this easier by putting $TAG under env and just updating that when we want to update the version.

I'll pull your changes and make the modifications for this. Thanks!

[garyb]

Ah sure, that makes sense. I'd been making this change in the core libraries (which I feel probably should be tested against the latest version) and just noticed builds were enabled for argonaut in travis too so thought I'd pass it on.