Installing Jekyll 2.5 on Heroku isn’t very dificult. You just have to find a working manual.
First follow this tutorial: How to deploy jekyll site to heroku
After that you can create a normal Heroku Ruby app without even using a custom buildpack.
Fix fucked up buildpack
If you allready followed any of the popular tutorials, you’ll have a custom buildpack set in Heroku. To reset the buildpack, run this command: