Code style updates, test updates (#39)

* changed from travis to git action

* used new env setting method

* fixed brackets

* updated dependencies

* updated dependencies

* updated dependencies

* updated unit tests

* updated MandrillTest.php added runtime directory creation

* fixed metadata class var
code style updates
added ext-fileinfo to composer.json

* tests with key from secrets

* tests with key from secrets 2

* tests with key from secrets 3

* fixed response and error handling

* fixed logging

* replaced sendRaw with send

* set global merge vars and template content independent from template language
fixed response handling

* fixed test

* fixed message test