Add semver 2 versioning in dependabot common

[amazimbe]

What are you trying to accomplish?

Implement semver 2 (https://semver.org/spec/v2.0.0.html) as the standard that dependabot follows for ecosystems that have no version standards; docker for example.

We have numerous errors and issues due to lack of clarity on what a valid dependency version should be and how to consistently compare versions within an ecosystem. Gem::Version has been inadequate so far and we think semver 2 will be an improvement especially for ecosystems without a well defined standard.

Malformed version errors and issues related to dependabot opening PRs to update a dependency to an unacceptable version, for example, a prerelease or beta release.

Anything you want to highlight for special attention from reviewers?

This one is the first in a series of PRs. Subsequent PRs will focus on improving ecosystem specific versioning by implementing the standard for the ecosystem where one exists; implementing deviations from semver 2 where applicable and improving test coverage which is below 50% in most cases.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[jakecoffman]

Can you write more tests with known valid/invalid values to make sure this implementation supports valid SemVer 2.0? For example, some folks have listed known values here: semver/semver#833

[amazimbe]

I'm very happy with the code and branch test coverage both of which are now on 💯 %