Add dependencies file so that all examples run on binder

[hydrosquall]

Changelog

• Motivation: 2 of the example binders wouldn't work as-is because the Docker image that was built for them was missing a few python dependencies
• Fix: I added a binder/requirements.txt to separate binder example dependencies from the requirements.txt associated with installation of the cartoframes module. I tested

Link to test: https://mybinder.org/v2/gh/hydrosquall/cartoframes/feature/add-binder-dependencies?filepath=examples

Hopefully this fix makes it easier for more people to try cartoframes without having to install anything locally. Separately, it looks like the failing Travis build is unrelated to this change, so I'm happy to rebase this branch at any point.

cc @andy-esch or any other project maintainers

[andy-esch]

Hey @hydrosquall, thanks for this, this definitely is an improvement :) I see other repos have a binder/ directory with the requirements.txt and postBuild files, but I don't see it mentioned in binder's docs. Is it an undocumented feature?

BTW, For external PRs we first merge them into develop, then do a PR against master from there. I'll ping you on that PR.

[hydrosquall]

@andy-esch : it's a feature that changed as of jupyterhub/binderhub#157 , but you're right that it seems to be undocumented. Would you be interested in filing a documentation bug on the binderhub repo? If not, I can do that next time I pass through.

It's something that popped up when I filed jvns/pandas-cookbook#62, because I didn't want the binder specific requirements.txt / Dockerfile to conflict with the pre-existing files with those same names.

Lastly, no worries! I've changed the base branch to develop

[andy-esch]

No worries, all good as far as I'm concerned!