[php] Set required PHP 7.1 in code comments and readme

[ybelenko]

There was PHP 5 appearances left in some places.
Travis-CI config has been slightly updated.
PHP_CodeSniffer has been removed in favor of PHP-CS-Fixer.

Related issue #1246

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

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.

[wing328]

Can you please update the samples under samples/openapi3 as well?

	modified:   samples/openapi3/client/petstore/php/OpenAPIClient-php/.travis.yml
 
	modified:   samples/openapi3/client/petstore/php/OpenAPIClient-php/README.md
 
	modified:   samples/openapi3/client/petstore/php/OpenAPIClient-php/composer.json
 
	modified:   samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/AnotherFakeApi.php
 
	modified:   samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/Api/DefaultApi.ph

Please also merge the latest master into this branch as the Circle CI failure has been fixed in the master.