Bum deps, add .formatter.exs and test_with_mocks shortcut.

[stjhimy]

• Bump dependencies - Fix error compiling certifi on tests

Tests are currently fail without updating certifi.

• Add .formatter.exs, run 'mix format'

This uses mix format to format the code base, also adds a .formatter.exs

• Add shortcut test_with_mocks

I'm always using this implementation, might be useful to someone.
If the last commit is not useful let me know and I will remove it.

[Olshansk]

Thanks @stjhimy!

I briefly looked through the changes (didn't test them myself) but glad to see this repo getting some new external contributions!