Phalcon v4 [Iteration 2]

[x] Change domain to phalcon.io
[x] Add license to header in .php files
[x] Rename namespaces from Niden to Phalcon\Api
[x] Change usage of Phalcon\Plugin to Injectable
[x] Review rest of comments and suggestions in comments of #22 PR
[x] Return back usage of use Phalcon\Filter; (https://github.com/phalcon/phalcon-api/pull/22#discussion_r322131277)
[x] Psalm integration with (less strict) level 8 of checks
[x] PSR-2 (in code)
[x] Bug fixes (if any)
[x] Minor code refactors
[x] Refactor tests/unit/BootstrapCest.php by moving into bootstrap or/and module initialization
[x] Fix env CODECEPTION_* variables
[x] Adapt Cache Service Provider to get adapter from config and not work only with memcached
[x] See if envValue() function is needed
[x] ~Create conditional import of Phalcon\Filter for 4.0.0-beta.2~
[x] ~Travis integration~ (#25)
[x] ~Scrutinizer CI fix/reboot~ (#26)

phalcon / rest-api