Separate installing vendors by composer use official image of composer

.travis.yml

@@ -8,6 +8,9 @@ php:
 services:
   - docker
 
+before_script:
+  - docker pull composer
+  - docker run --rm -v $(pwd):/app composer install --prefer-source --no-interaction
 install:
   - docker version
   - sudo pip install docker-compose

docker-compose.yml

@@ -11,7 +11,7 @@ services:
         working_dir: /code
         environment:
           PHP_VERSION: ${PHP_VERSION}
-        command: bash -c "composer install --prefer-source --no-interaction && php ./vendor/bin/phpunit"
+        command: bash -c "php ./vendor/bin/phpunit"
         depends_on:
           - mysql
           - mongodb