Use Poetry for dependency management and deployments

[maebert]

Since jrnl was first written in 2012, a lot has happened in the wonderful world of dependency management and python deployments. This PR moves the deployment process to Poetry, which has the following advantages over pip or pipenv

• Requirements can be locked (similar Ruby gems or Yarn), creating less dependency conflicts for users and developers
• Poetry handles virtual environments, no need to create and maintain one yourself
• It's super easy to publish from Travis straight to PyPi (set to deploy from master on tagged commits)

With poetry, the new development workflow is

# 1. Set up environment
git clone ...
cd jrnl
poetry install

# 2. Add new dependencies
poetry add my-dependency

# 3. Run tests (or any other command that requires the installed dependencies)
poetry run behave

I'll update the documentation for this after #611 is merged.

Also fixes #610.

[wren]

This is a good update, but it changes a few dependencies. I think that introduces risk to v2.0 since we already have people testing, and I don't want to lose that momentum. I'd love to get this merged in first thing for v2.1, though.

[maebert]

Yup, agree, let's get 2.0 in first and then rebase this against master.

[lock]

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