git problem

$ git push origin master
To git@github.com:rakiru/bMiniMap.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'git@github.com:rakiru/bMiniMap.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.