Remove .travis.yml instructions

I'm assuming the link was meant to point people to the `before_script` which was removed in https://github.com/jonleighton/poltergeist/commit/b931466f4f269ab9d3e93ed94292ae5462228f67 .

If the instructions are still needed, the `before_script` source could be added to the README as a code block, or it could link to an older `.travis.yml` version.