Remove whitespace from .travis.yml

Previous change has caused Travis to complain 'Please override the
script: key in your .travis.yml to run tests'.
https://travis-ci.org/tangentlabs/django-oscar/jobs/35319191

I am though so I'm confused. I did tinker with the whitespace last time
so here I adjust it in the hope that it fixes things. (I have no idea
what I'm doing).