Error ! [rejected] master -> master (non-fast-forward) on Heroku/Git

$ git push origin master
# To https://github.com/user/repo.git
#  ! [rejected]        master -> master (non-fast-forward)
# error: failed to push some refs to 'https://github.com/user/repo.git'
# To prevent you from losing history, non-fast-forward updates were rejected
# Merge the remote changes (e.g. 'git pull') before pushing again.  See the
# 'Note about fast-forwards' section of 'git push --help' for details.

This error can be a bit overwhelming at first, do not fear. 
Simply put, git push -f https//github.com/user/repo.git to force the upload.