Update .travis.yml to use a specific version of composer

Author: hmerritt

.travis.yml

@@ -4,12 +4,12 @@ php:
   - 7.3
 
 install:
-  - composer self-update
-  - composer install --dev --no-interaction
+  - composer self-update 1.9.3
+  - composer install --no-interaction
 
 script:
   - mkdir -p build/logs
   - php vendor/bin/phpunit --coverage-clover build/logs/clover.xml
 
 after_script:
-  - travis_retry php vendor/bin/php-coveralls -v
+  - travis_retry php vendor/bin/php-coveralls -v