Add keys.js to global .gitignore

[jruedas1]

This is not an error as such, but a suggestion. AFAIK you never want to push a keys.js file to Github. Students forgetting to add keys.js to their .gitignore file is a frequent situation, which often requires some complex git wizardry like git rebase -i to clean up commit histories and totally remove the key information from version control. I propose it would be better to proactively add keys.js to the global gitignore file. If a situation ever arises where a student actually wants to include it in version control, it's easier to edit the global gitignore than it is to edit the git commit history.