Binder doesn't work due to old branch names

[paulmaunders]

I used generate_badge() to add a binder badge to my repo, however when clicking it binder gave the following error:

Could not resolve ref for gh:paulmaunders/coronavirus-graphs/master. Double check your URL. GitHub recently changed default branches from "master"to "main". Did you mean the "main" branch?

Updating the badge link to main helped, but then I encountered another error:

Resolving github.com (github.com)... 140.82.114.4
Connecting to github.com (github.com)|140.82.114.4|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2020-12-31 17:43:32 ERROR 404: Not Found.

So it looks like the references to master in the DockerFile need updating too.

[paulmaunders]

I've just realised there is a pull request to allow setting of branch names: #61

This is a good idea, but I suggest the default branch name is updated to main as well.

[karthik]

Thanks, I'll merge that shortly.

[paezha]

Thank you for this awesome package! I am new to binder and could set my repository up with minimal pain. That said, I had the same issue with the badge making reference to the master branch. I edited it to main and now the link works, but this might throw off novice users.