php 7.4 compatibility check list

[Grummfy]

it's time to wake up a bit and check some PHP 7.4 compatibility

PR:

• PHP 7.4 compatibility #825 composer update
• fix some issue related to string casting warning
• Add 7.4 to travis #830 add travis build
• Fix deprecated. unparenthesized (php 7.4) #835 fix deprecated unparenthesized
• php 7.4 : includer fail #837 includer test fail
• php 7.4 failed test because of error #838 error on unit test

TO check

• mock generation with typed propertie
• mock generation with covariant https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters
• preloading activation/disactivation & mock generation https://wiki.php.net/rfc/preload
• does we need to add some asserter for the custom serialization attributes https://wiki.php.net/rfc/custom_object_serialization ?
• does we need to add some asserter for the ffi (see https://wiki.php.net/rfc/ffi ) ?