Upgrade to Guzzle version 7

• no longer depends on Guzzle 6
• fixes #11

2.0.0

B/C breaking changes

• Renamed Piwik to Matomo.

Bugfixes

• #3 Responses with an empty or non-json body are errors, even if they return 200 OK.

1.0.1

Bugfix release. Improves compatibility with PHP 5.

• #2 by @claudiu-cristea - PHP5 compatibility: Only starting with PHP7 keywords (such as 'unset') can be used as method names

1.0.0

Initial release.