Fixes #1485 Zeus should not be bundled

Fixes #1485 Zeus should not be bundled.

According to [`zeus` README](https://github.com/burke/zeus/blob/master/README.md), zeus should not be bundled.

> Q: "I should put it in my Gemfile, right?"

> A: No. You can, but running bundle exec zeus instead of zeus adds precious seconds to commands that otherwise would be quite a bit faster. Zeus was built to be run from outside of bundler.