Ok, well I was trying to avoid opening Ps sine I was still in my jamies and in a comfortable position in bed watching tv but since you're such a sweetheart I'll do it.
As long as I'm in Ps I went a little further then just circling it. Hope you don't mind.

Here is what I'm talking about, these pixelated areas in various part of the render.
Like I said this is probably due to poor rendering or just the over all quality of the render, both of which are not your fault because I know sometimes I fall in love with an image only to realize that it's not the best of quality and you can't seem to find a better version.
In you're case it's only the edges that are of big concern so you can fix it with a little bit of blending. So make friends with the blur, burn, dodge,and pentool. Yes I said pentool sounds odd but it's my baby and I use it for everything. I would make love to it if I could and how sweet it would be but enough babbling from me, moving along.
ok, what I did;
1. I blurred areas I felt needed blurring with the blur tool set on low opacity.
2. took my pentool, set on "shape layers"(top left corner), and made an almost mimiced shaped around the two areas I circles earlier just a tad larger. Use the eye drop tool to extract the same colour as the hair but it was just a normal black this time.
3. Zoom in and smudge your render in with your BG for a more seamless flow. Keep the smudge tool on a low opacity and use a small hard brush
4. Burn areas you feel need darkening and lighten areas with the dodge tool your feel lightening. All this is based on your lighting and where it's coming from.
4. last sharpen the image and lower the opacity a bit.

My end product. If you ask me " what?" again I swear I'll stick a fork in you

. Well maybe not a fork a spoon at most but you have my word it will be an interesting experience.
Hope I helped. Peace.