Add exponential bucket helper

Fix incorrect miniumum version of php

Upgrade min. version of php to 7.1, upgrade support for phpunit and f…

…ix tests

Release v0.9.1

    * Add Redis auth support
    * Fix incorrect cast after json_decode

Merge pull request endclothing#36 from krukru/InMemory-impl

In memory impl

Merge pull request endclothing#32 from Jimdo/get_or_register

Add getOrRegister methods

Merge pull request endclothing#28 from Jimdo/pushgateway

Add PushGateway implementation

Add support for APC

Merge pull request endclothing#22 from Jimdo/not_found_exception

Make metrics registration/retrieval more robust

Merge pull request endclothing#21 from Jimdo/forbid-to-register-metri…

…cs-twice

Forbid registering the same metrics twice