Speed up composer install on travis

Add caching between builds

Author: ihor-sviziev

.travis.yml

@@ -6,6 +6,9 @@ php:
   - 7.2
   - 7.3
   - 7.4
+cache:
+  directories:
+    - $HOME/.composer/cache/files
 install: composer install --no-interaction --prefer-dist
 script:
   - vendor/bin/phpunit