Use updated papermill v0.16 API

[matthewfeickert]

Description

Use the updated papermill v0.16 API that uses output_path instead of output. Also require papermill to be compatible with v0.16 by using PEP 440/508's compatible version syntax. This ensures that the API should be stable as it will remain at v0.Y versions even after major release v1.Y.Z is released.

This also has the nice feature of being able to intentionally choose when to update to a newer backend API even if the backend has had a new major release with API breaking changes for a long time.

Noting that if the release is specified down to the bug fix release (X.Y.Z) then it will be pinned down to the minor release (X.Y). c.f. PEP 404:

the following groups of version clauses are equivalent:

~= 1.4.5
>= 1.4.5, == 1.4.*

so as updates from other libraries are desirable, as long as there is no API breaking changes, then all dependencies should be specified as

~= X.Y

unless they need to be specified all the way down to X.Y.*

c.f.

• https://www.python.org/dev/peps/pep-0440/#compatible-release
• https://www.python.org/dev/peps/pep-0508

- Use updated papermill `v0.16` API
- Require papermill version compatible with v0.16 by using PEP 440/508's compatible version syntax
- Add pip freeze to Travis to make viewing the environment easier