updating the dependencies

Just converting to the tild opeartor which is now suppose to be the default one.

Is there a reason not to include the composer.lock file ?
It's not completely following the semver but as he [explain](https://github.com/michelf/php-markdown#public-api-and-versioning-policy) it's only relevant for people that use his api to create other parser on top of his not for people like us that just use it ( it being the public member of his api).
And we only use it at one place https://github.com/phansible/phansible/blob/master/src/Phansible/Controller/DefaultController.php#L43.