Core: Fix Drupal coding standard check on Scrutinizer

A process is described at
https://scrutinizer-ci.com/docs/tools/php/code-sniffer/#problems-with-drupal-coding-standard
but that description does not work for us. Problems regarding a
loading Sleevomat classes remain.

Bases on the version history for the drupal/coder package we can tell
that this dependency was introduced in version 8.3.14 of drupal/coder
which was recently released. Scrutinizer will install version ^8.3 by
default.

To address this we pin coder to the last known working version.

When using this approach third party packages will end up in
./vendor. We do not want to include this in our analysis so we
configure PHPCS to ignore them.

.scrutinizer.yml

@@ -1,6 +1,9 @@
 build:
   nodes:
     analysis:
+      dependencies:
+        after:
+          - composer require --dev drupal/coder:8.3.13
       tests:
         override:
           - phpcs-run

phpcs.xml.dist

@@ -3,6 +3,7 @@
   <description>PHP CodeSniffer configuration ding2.</description>
   <!-- Check all files in the current directory and below. -->
   <file>.</file>
+  <exclude-pattern>vendor/*</exclude-pattern>
   <arg name="extensions" value="php,module,inc,install,test,profile,theme,css,info,txt,md"/>
   <config name="drupal_core_version" value="7"/>
   <rule ref="Drupal"/>