Update git submodules

* Update Wikibase from branch 'REL1_35'
  to 03aa0fc7d465d924721d48170de73263946d1bd6
  - Prevent Composer from updating to v2 in Travis CI
    
    The composer merge plugin doesn't yet work with composer 2.
    
    See wikimedia/composer-merge-plugin#184
    and https://phabricator.wikimedia.org/T248908
    
    Bug: T266459
    Change-Id: I6d78bdb05a518778c958a9b47b053b4e64f7e26d
    (cherry picked from commit c5422556ba1b3d18c5a17ad1aa8da26d6d745993)