Check for updated dependencies

[osma]

Skosmos depends on a number of PHP and JavaScript libraries.
We should check if there are more recent versions of these available and upgrade the dependencies if possible. If there are problems upgrading some libraries, we could open separate issues for them.

[osma]

SensioLabs Insight claims that the current Twig version we use has a security issue:
https://symfony.com/blog/twig-sandbox-information-disclosure

This doesn't seem to be actually critical for Skosmos since we don't use the sandbox feature (and don't have untrusted templates), but getting rid of the complaint would be nice nevertheless.