Travis build

I was fighting travis yesterday a lot, so I thought I could help here. This makes travis builds pass for 1.8.7, 2.0.0 and ruby-head (check [here](https://travis-ci.org/deivid-rodriguez/columnize). It forces rdoc and rake to be installed using bundler and specifiying them as dependencies in gemspec. Somehow one of the commits got split into two... sorry.

Hope it helps.