Add script to sync the Binder environment package versions with the requirements.txt files'

[jhlegarreta]

Add a script that keeps the package versions in the requirements.txt file and the Binder instance in its environment.yml file in sync.

It could be a Python script that reads the versions of each package in the requirements.txt file and uses a regex to bump the ones in the binder environtment.yml file.

Related to issue #226.

[jhlegarreta]

May not be necessary:
https://mybinder.readthedocs.io/en/latest/using/config_files.html#requirements-txt-install-a-python-environment

We may get away with it copying the top-level requirements.txt file, or making Binder use the top-level one if possible.

[jhlegarreta]

On a slightly related note, testing that Binder is correctly launched can also be relevant:
https://discourse.jupyter.org/t/how-to-build-mybinder-image-automatically-at-each-commit/5381