Upgrade PHPUnit to version 5

- Fix deprecated getMock
- Use PHPUnit 4 for PHP 5.5 build

Manually cherry-picked from PR
https://github.com/wallabag/wallabag/pull/2201

.travis.yml

@@ -55,6 +55,7 @@ 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 [[ $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini; fi;
+    - if [ "$TRAVIS_PHP_VERSION" = "5.5" ]; then composer require "phpunit/phpunit:4.*" --no-update; fi;
     - composer self-update --no-progress
     - if [[ "$DB" = "pgsql" ]]; then psql -c 'create database wallabag_test;' -U postgres; fi;