Replace TravisCI with GitHub Actions

[flavioheleno]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	none
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

This PR replaces TravisCI with GitHub Actions as mentioned here.

Why?

TravisCI support for Open Source projects have drastically changed in 2020 as can be seen below.

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• https://docs.travis-ci.com/user/migrate/open-source-repository-migration/#frequently-asked-questions

Example Usage

Once this PR gets merged, simply go to the Actions Link to see the results.

A sample execution can be seen here

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

^ This PR does not change the library itself and I could not find anything like that on the CHANGELOG.md, therefore I have updated it nor documented it anywhere else to be honest.

To Do

• Check if the integration with scrutinizer-ci.com is still functional

[flavioheleno]

@joelwurtz is this what you had in mind?

[dbu]

thanks! this has been wrapped up in #63