Fix casperjs tests

As of version `1.1-beta3` the `test` subcommand pre-defines a _casperjs_ instance
and refuses to execute and exits with:
```
Fatal: you can't override the preconfigured casper instance in a test environment.
```
See also the [Docs](http://docs.casperjs.org/en/latest/testing.html#test-command-args-and-options)