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)
Interesting point. That should probably go into them Gemfile then (which is only for development and can be tied to a specific platform).

I imagine there won't be many new users on Ruby 1.8 - it feels like more trouble than it's worth to add a special error message just for this=