Fix tests

Ensure tests after the InstallCommand test will have a clean database.
Previous attempt failed because of a foreign key

Also, nightly on Travis seems to target PHP 7.2-dev, so I put 7.1 in requirement and moved nightly in allowed failures.
And it seems xdebug isn't shipped with PHP 7.1, so I added a workaround to avoid build to fail when trying to remove xdebug.ini file.

.travis.yml

@@ -20,6 +20,7 @@ php:
     - 5.5
     - 5.6
     - 7.0
+    - 7.1
     - nightly
     - hhvm
 
@@ -38,6 +39,7 @@ matrix:
           env: DB=pgsql  # driver for PostgreSQL currently unsupported by HHVM, requires 3rd party dependency
     allow_failures:
         - php: hhvm
+        - php: nightly
 
 # exclude v1 branches
 branches:
@@ -46,7 +48,8 @@ branches:
 
 before_script:
     - if [[ $TRAVIS_PHP_VERSION != hhvm ]]; then echo "memory_limit=-1" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini; fi;
-    - if [[ $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini; fi;
+    # xdebug isn't enable for PHP 7.1
+    - if [[ $TRAVIS_PHP_VERSION != '7.1' && $TRAVIS_PHP_VERSION != 'hhvm' ]]; then phpenv config-rm xdebug.ini; fi
     - composer self-update --no-progress
     - if [[ "$DB" = "pgsql" ]]; then psql -c 'create database wallabag_test;' -U postgres; fi;