Fix Travis CI

[kubawerlos]

Minimal changes to make Travis CI work:

• Ubuntu Xenial does not support PHP 5.4 and 5.5 and since it's the default environment we have to define Trusty in config
• increase memory limit for composer
• upgrading mockery to stable version to fix PHP Fatal error: Declaration of Mockery_2_Nette_DI_ServiceDefinition::__isset($name) must be compatible with Nette\DI\ServiceDefinition::__isset(string $name): bool in vendor/mockery/mockery/library/Mockery/Loader/EvalLoader.php(16) : eval()'d code on line 25

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In separate PR:
composer self-update is made by Travis CI itself, no need to do it 2nd time
disabling Xdebug - not needed and slows down - especially 5.x - jobs
using Prestissimo to speed up jobs

[OskarStark]

Could you please explain why this fixes Travis? Just for the record, thanks 👌🏻

[kubawerlos]

@OskarStark Travis is green 🍾