Keep composer cache in CI cache, instead of installed vendor directory

Benefits:

• When a branch changes the contents of vendors (updates or adds dependency in composer.lock), composer has to re-download everything that changed. That appends a lot with multiple branches having a distinct composer.lock
• If the composer.lock is not committed, composer have to download json manifests from packagist. Keeping the cache of these files offloads packagist and speedup dependency resolution.

Downsides:

• composer have to extract the cached archives into vendor directory on each build (a lot faster that an actual download)
• cache directory grows with each dependency version and may need to be cleaned.

Documentation will be updated once this PR get merged. https://docs.gitlab.com/ce/ci/examples/php.html#using-composer