Upgrade interfaces from php56

[cawolf]

Short description of what this PR does:

• ensures interfaces are 7.1 compatible
• adds phpstan/phpstan to travis
• fixes 3 phpstan issues

Closes #94

Hint: I had to disable the treatPhpDocTypesAsCertain option for phpstan, otherwise it would complain about an unreachable code block in Reference.php.

[jcchavezs]

Merging as it is, thanks @cawolf