Closes #628

Ignore closure return types, as we need to support PHP < 7.

This is not really workable. Need to devise another way to run psalm
periodically without keeping it breaking the build.

This prevents a lot of -dev dependencies in composer.lock which are
not used otherwise and generate 'scary' warnings like "Package x is
abandoned".

Closes #718

We've just been moved from Precise to Trusty, and PHP 5.3 is not available in the latter, so builds fail.

The segfault happens in the coverage report. Since we send coverage
info only with PHP 5.6, avoid running it entirely, saves buildtime aswell.

Try to make it build with the saml module included by using the phpunit version we are requiring instead of the one preinstalled.

Calling composer with "update" instead of "install" should update the versions referenced in the composer.lock file, instead of blindly trying to install them, leading to conflicts.

Try with "composer update" instead of "composer install". The latter does not support --prefer-lowest.

Since this branch is now officially released we need to support it.

This add testing on PHP 7.0 (nightly build). The test is added as an
allowed failure for now. but it can be useful to keep an eye on it just
in case we do something that won't work in PHP 7.