Replace abandoned Jimdo/prometheus_client_php dependency

[adi64]

From the issue #11:

https://github.com/Jimdo/prometheus_client_php is archived and no longer maintained.
They didn't add the abandoned value in composer.json, so there's no link to it from there, but they linked to this repository in the readme: https://github.com/endclothing/prometheus_client_php .

This changes the dependency accordingly.

This will bump the required minimum PHP Version to 7.1.

Closes #11

[Satariall]

@bradsuperbalist please make an attention to this PR. The jimdo/prometheus_client_php package marked as abandoned from now and this blocking install laravel-prometheus-exporter library.

[FutureMatt]

https://github.com/endclothing/prometheus_client_php has a minimum requirement of PHP 7.2, in order for this to work the minimum requirement for PHP in composer.json needs to be bumped to ^7.2 at least. Also the .travis.yml file will need to be updated to remove the 5.6, 7.0 and 7.1 versions in order for the tests to pass.

[adi64]

Closing this in favor of #22. Sorry I neglected this MR.