Merge branch 'add/linting-master' into 'master'

Add linting

See merge request !8
5 jobs for master in 4 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #3910
PHPunit:PHP5.3:MySQL

00:03:08

passed #3911
PHPunit:PHP5.6:MySQL

00:02:54

passed #3912
PHPunit:PHP7.0:MySQL

00:02:49

passed #3913
PHPunit:PHP7.1:MySQL

00:02:44

passed #3914
PHPunit:PHP7.2:MySQL

00:04:58

 
Name Stage Failure
failed
PHPunit:PHP5.3:MySQL Test
No syntax errors detected in ./tests/bootstrap.php
No syntax errors detected in ./tests/test-sample.php
$ phpcs -n
...

Time: 116ms; Memory: 22Mb

$ phpunit
Your server is running PHP version 5.3.29 but WordPress 5.2.1 requires at least 5.6.20.
ERROR: Job failed: exit code 1