Upgrade to new maintance rules

[medikoo]

• Fix CI to actually run tests
• Bring back Node.js v6 support (accidentally removed with v2.3.3)
• Rely on @serverless/eslint-config
• Enforce Prettier formatting
• Update style in all files to conform to newly introduced rules
• Enforce semantic commit messages
• Introduce semi-automated release process
• Update Travis CI with release automation
• Remove Node.js v4 from CI setup (it was not supported for very long time)
• Cleanup .gitignore
• Remove configuration files specific to developer environment

[pmuens]

Good stuff! 🙏 Thanks @medikoo 👍

I just added one comment regarding a core dependency (we might want to double check if the plugin still works).

[pmuens]

Is this breaking since it's a downgrade?

[medikoo]

I don't think so. All tests pass.

Also upgrade from v32 to v41 was made without any extra changes to code: https://github.com/serverless/serverless-google-cloudfunctions/pull/176/files

And that was actually breaking (as it dropped Node.js v6 support, CI didn't pick that as its setup was broken - just lint was run, no tests).

So this is an attempt to bring back support for Node.js v6 in v2 scope, which we will drop with v3

[pmuens]

Makes sense. Thanks for the clarification 👍