Set composer version 2.2

for the compatibility of older(<7.2) PHP version

Author: koriym

.github/workflows/continuous-integration.yml

@@ -15,7 +15,6 @@ jobs:
         operating-system:
           - ubuntu-latest
         php-version:
-          - '5.5'
           - '5.6'
           - '7.0'
           - '7.1'
@@ -34,7 +33,7 @@ jobs:
         with:
           php-version: ${{ matrix.php-version }}
           coverage: xdebug
-          tools: none
+          tools: composer:2.2
           ini-values: assert.exception=1, zend.assertions=1
 
       - name: Get composer cache directory