Maintain seperate git branches - One for heroku and another for github

I have got a project in nodejs which is on github and deployed on heroku. In that project I use a few API keys and those keys are stored in config.js file which is ignored while pushing to github. But I need those files for deployment on heroku. So my problem is when ever I make any changes to push on github I have to ignore that config.js file and when I deploy on heroku I have to include those again in my branch. I am stuck in this viscous cycle.

Thank You

This is my second time asking question here. So if I did not follow any convention please tell so that I may edit for future visitors.