User composer v1 in travis

.travis.yml

@@ -66,6 +66,7 @@ matrix:
       env: EXTRA_DEPS=phpHigh  PRESTASHOP_TEST_TYPE=unit
 
 before_install:
+  - composer self-update --1
   - /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_10.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :10 -ac -screen 0 1600x1200x16
   # Avoid Composer authentication issues
   - if [[ "$TRAVIS_REPO_SLUG" = PrestaShop/PrestaShop ]]; then cp travis-scripts/.composer-auth.json ~/.composer/auth.json; fi;