[php-slim4] Set required PHP version to 7.2

[ybelenko]

Update Slim4 package and rest dependencies to meet PHP 7.2 version.
I've tested build locally with PHP 7.2 and 7.3, there are warnings from PHPUnit but no critical errors or test fails.
Related issue #6522

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon

[ybelenko]

Travis CI fails because configured to old PHP 7.1 environment:

openapi-generator/.travis.yml

Line 88 in e2e3405

- phpenv global 7.1