Tags and product type V2 support and minor fixes
Added api and entity for the V2 product types and tags.
Fixed the typo in the V2 Versions entity for the productTypes value.
Raised minimum PHP version requirement to 7.1 from 5.6. This was effectively a requirement before as well, it was just not reflected in composer.json and the travis config.
Removed PHP 5.6 and 7.0 travis tests, added PHP 7.2 and 7.3.