Quote all version numbers in the CI configuration

This makes our usage of version numbers in the configuration file
consistent. It also ensures that no version number will get treated
as a number and rounded (e.g., 8.0 would have gotten rounded to 8).

Author: oliverklee

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
         php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3' ]
         coverage: [ 'none' ]
         include:
-          - php-version: 7.4
+          - php-version: '7.4'
             coverage: xdebug
 
     steps:
@@ -91,11 +91,11 @@ jobs:
       matrix:
         include:
           - command: sniffer
-            php-version: 7.4
+            php-version: '7.4'
           - command: fixer
-            php-version: 7.4
+            php-version: '7.4'
           - command: stan
-            php-version: 7.4
+            php-version: '7.4'
 
     steps:
       - name: Checkout