Add ruby 1.8 to travis

This pull request is to make sure we have working ruby 1.8 support. I expect the build to fail initially, until I can see what's broken and fix it (I don't have a working version of ruby 1.8 on my development machine)
Sadly, I don't think this works the way you might expect. This will add the dependency to the built gem based on the ruby version used _when building the gem_, not based on the user's ruby.

AFAIK there's no reasonable way to do conditional dependencies like this; we would either need to make it a 100%-all-the-time dependency, or rescue the inevitable LoadError on 1.8 and just explain what went wrong to the user.