Use public_repo instead of repo when generating OAuth token

`repo' allows access to both public and private repositories,
whereas `public_repo' allows only public repositories.
Fix broken command in README
Merge pull request #159 from flacjacket/fix_readme

Fix broken command in README
Merge pull request #151 from mattpap/public_repo

Use public_repo instead of repo when generating OAuth token

There is no reason for SymPy Bot to have access to private repositories.