Install PhantomJS 2.1.1 on Travis-CI

I've found it to be much more stable locally - hopefully it will help
with our flaky CI issues (#901).

Code mostly cargo-culted from
https://github.com/mitchlloyd/ember-orbit/commit/9d3afe3d0434d9e3572e018d65ea96857fe5fa25
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f