If you think using a plugin for Jekyll to set some variables in the head is unnecessary and is making things very untransparent. I would advice to use (a variation of) this code:
Note: I really like the SEO plugin for Jekyll. However, I think we should encourage people to understand the code they are using. Hiding the code in a plugin is not going to contribute to that goal. WordPress feels to much like a black-box. Jekyll should not.