Update version key in config file after version changes

[jonakeys]

This PR fixes #1638
As mentioned in #1638 the reference states:

version
jrnl automatically updates this field to the version that it is running. There is no need to change this field manually.

jrnl now checks if the version in the config file differs from the current version, and if so, updates it. And I also added a BDD test for this functionality.

Side question: While working on this, I saw no BDD test for adding missing keys in the config. But for some reason the message 'Configuration updated to newest version at' prints when testing in 'poetry shell', but not while running in a test. Do BDD tests automatically fill in the config fields that aren't there yet?

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[jonakeys]

In given_steps.py I found:

#Add jrnl version to file for 2.x journals
with open(config_dest, "a") as cf:
cf.write("version: {}".format(version))

So that's how the missing version-key gets added.

[micahellison]

Thank you! No notes, looks great.

[wren]

Looks great. Thank you!