[Travis] Add more environment of tests

.travis.yml

@@ -9,9 +9,29 @@ cache:
 matrix:
     include:
         - php: 7.1
+          env: COMPOSER_VERSION="1.6.*"
+        - php: 7.1
+          env: COMPOSER_VERSION="1.7.*"
+        - php: 7.1
+          env: COMPOSER_VERSION="1.8.*"
+        - php: 7.2
+          env: COMPOSER_VERSION="1.6.*"
+        - php: 7.2
+          env: COMPOSER_VERSION="1.7.*"
         - php: 7.2
+          env: COMPOSER_VERSION="1.8.*"
         - php: 7.3
+          env: COMPOSER_VERSION="1.6.*"
+        - php: 7.3
+          env: COMPOSER_VERSION="1.7.*"
+        - php: 7.3
+          env: COMPOSER_VERSION="1.8.*"
+        - php: nightly
+          env: COMPOSER_VERSION="1.6.*"
+        - php: nightly
+          env: COMPOSER_VERSION="1.7.*"
         - php: nightly
+          env: COMPOSER_VERSION="1.8.*"
 
     allow_failures:
         - php: nightly
@@ -21,7 +41,9 @@ matrix:
 before_script:
     - |
       composer global require --prefer-source phpunit/phpunit:"^6.5.0" php-coveralls/php-coveralls:^2.1.0
-      composer install --prefer-source
+      if [ "$COMPOSER_VERSION" == "" ]; then composer install --prefer-source; fi;
+      if [ "$COMPOSER_VERSION" != "" ]; then composer require "composer/composer:${COMPOSER_VERSION}" --no-update; fi;
+      if [ "$COMPOSER_VERSION" != "" ]; then composer update --prefer-source; fi;
       mkdir -p ./build/logs
 
 script: