Fix authentication for PR by external contributors

.travis.yml

@@ -47,6 +47,9 @@ before_script:
     - if [ -n "$GH_TOKEN" ]; then composer config github-oauth.github.com ${GH_TOKEN}; fi;
     - if [[ "$DB" = "pgsql" ]]; then psql -c 'create database wallabag;' -U postgres; fi;
 
+before_install:
+    - if [[ $TRAVIS_REPO_SLUG = wallabag/wallabag ]]; then cp .composer-auth.json ~/.composer/auth.json; fi;
+
 script:
     - travis_wait composer update --no-interaction --no-progress
     - ant prepare-$DB