This is the last version which supports Ruby 2.3

Migrate CI from Travis CI to GitHub Actions

* Update actions/checkout to v4

* update CHANGELOG.md

* fix .github/workflows/check_changelog.yml

* Update Gemspec versions

There's no reason to block pessimistically (`~>`) to any of these versions. Also add dependencies (bigdecimal) which is warned about in a deprecation warning:

```
/Users/rschneeman/.gem/ruby/3.3.1/gems/get_process_mem-0.2.7/lib/get_process_mem.rb:2: warning: bigdecimal was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec. Also contact author of get_process_mem-0.2.7 to add bigdecimal into its gemspec.
```

* Switch to testing on modern Ruby versions

This doesn't directly mean that these versions won't work, but we no longer make claims about their support.

* Changelog

* Switch to standardrb for linting

* Apply standard linting styles

```
$ be standardrb --fix
standard: Use Ruby Standard Style (https://github.com/standardrb/standard)
  lib/puma_worker_killer/puma_memory.rb:78:20: Style/HashConversion: Prefer ary.to_h to Hash[ary].
standard: Run `standardrb --fix-unsafely` to DANGEROUSLY fix one problem.
```

* Fix uninitialized warning

```
/Users/rschneeman/Documents/projects/puma_worker_killer/puma_worker_killer.gemspec:17: warning: global variable `$INPUT_RECORD_SEPARATOR' not initialized
```

* v1.0.0

* Update url