Make compatible with phpunit 7

[luispabon]

• Bump phpunit dependency to ^7 - cannot support both 7 and 6 without breaking because of a method override that now has to return void.
• Also bump php version to 7.1
• This will need tagging as v3 because above
• Doc change requires creating a v2 branch, should be done after merging #8, or would need #8 merging into it instead of master