Update webonyx/graphql dependency for PHP 7.3

[mikefrancis]

Fixes #403

Also removes PHP 5.5, PHP 5.6 and PHP 7.0 from Travis as they are either:

• EOL
• the underlying webonyx/graphql-php package has a higher minimum requirement for PHP

[mfn]

The PHP 7.2 jobs look promising on Travis.

TBH I'm surprised that no code-adaption change is required ?!

[coveralls]

Coverage remained the same at 70.537% when pulling 1bc5d42 on mikefrancis:update-webonyx/graphql-php into 66e9233 on Folkloreatelier:develop.

[mikefrancis]

@mfn Have fixed the tests for PHP 7.1 and 7.2

[mfn]

Nice! I hope I found time during the holidays to test it with my application!

[mfn]

IMHO this needs to be adjusted, too. webonxy 0.13.0 requires 7.1+

[mfn]

Nice! I hope I found time during the holidays to test it with my application!

I already have to attest that I have to postpone this indefinitely.

I've upgraded webonyx locally since folklore lib doesn't need any code changes according to this PR.

However I've run into multiple incompatibilities regarding classes and function signatures I can't quickly fix. They are independent of Folklore but webonxy did refactor/adjust many things which can reach your application code nevertheless.

So, sorry, I can't give any feedback for now :|

[mikefrancis]

Closing this as I moved over to the rebing/graphql-laravel package which supports PHP 7.3