How do I uninstall this buildpack in Heroku?
In the current version of heroku (v3.1.0 as of this writing) you can use heroku buildpacks:remove [BUILDPACK_URL]
Typing heroku buildpacks help lists the available commands:
buildpacks:add BUILDPACK_URL # add new app buildpack, inserting into list of buildpacks if neccessary buildpacks:clear # clear all buildpacks set on the app buildpacks:remove [BUILDPACK_URL] # remove a buildpack set on the app buildpacks:set BUILDPACK_URL # set new app buildpack, overwriting into list of buildpacks if neccessary