Fix missing reference to requirements-base.txt

[GeorgianaElena]

Include requirements-base.txt in MANIFEST so it gets installed.
Fixes #502

[yuvipanda]

Thanks, @GeorgianaElena!

[betatim]

To catch errors with the manifest files we'd have to run our integration tests in/from a copy of the repo that was created by some kind of bundling tool right? I was surprised we didn't catch this in the integration tests and trying to understand why.

[GeorgianaElena]

My guess is that it has something to do with the checkout step in our circleci config.
Expand the checkout code in our tests to see what circleci is doing.

I believe because of this, even if we build tljh here without the requirements file lets say, it will still find the file because it has access to the source code. In a production scenario, this doesn't happen.

P.S. I'm not sure if I'm making any sense, this is just a hunch (it hard to understand how circleci works).

[betatim]

Thanks for investigating. Debugging CI setups is no fun :-/ Looks like #503 is a good place to track and work on this issue.