Merge pull request #2861 from wallabag/2.3-droping-5.5

Dropping PHP 5.5

diff --cc .travis.yml
Simple merge

diff --cc composer.json
Simple merge

diff --cc docs/de/user/installation.rst
Simple merge

diff --cc docs/en/user/installation.rst
Simple merge

diff --cc docs/fr/user/installation.rst
Simple merge