Travis: Don't test on Ruby 1.8 (it's deprecated, and it breaks the build)

The build is currently breaking because of an issue with Ruby 1.8, which is [officially retired](https://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/). This PR should fix the build by removing Ruby 1.8 support, as it only errors right now on Ruby 1.8.