@const is not an official part of PHPDoc

Pointed out by @GaryJones: https://github.com/tommcfarlin/WordPress-Plugin-Boilerplate/commit/4d629b53238b101f71e718f4c41de45997298a69#commitcomment-3943262
Replacing `$this->version` by the new class constant
Merge pull request #71 from GeertDD/const

Updates related to the new VERSION constant