Throwable is available starting with 7.0 PHP version

[kampalex]

Issue Summary

According to composer.json, this library supports PHP version 5.6 and greater.

During review, my IDE warned that Throwable is used in file InvalidRequest. This interface is available since PHP 7.0.

Solution: bump PHP version requirement to '>=7.x' or replace Throwable using Exception.

Technical details:

• php-http-client version: current master (according to composer.lock of other project: 3.10.6 / 2020-04-29T19:42:26+00:00)
• php version: 7.1 (compatible version for configured phpunit)

[kampalex]

Added context:
Luckily, the thrown InvalidRequest exception in makeRequest() does not have the $previous/3rd argument, so the null isn't compared against expected interface Throwable.
This is the reason build tests in PHP 5.6 are passing. But when set, it will lead to fatal error.

[childish-sambino]

Nice find. PR submitted.