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)
Sorry, my point was supposed to be that there won't be many new users, and I expect anyone who's already using the gem will have figured out that they need to require json already=