Add caching to CI build

Author: designcise

.travis.yml

@@ -1,10 +1,14 @@
+cache:
+  apt: true
+  directories:
+    - $HOME/.composer/cache/files
+
 language: php
 
 php:
   - 7.4.0
 
 before_script:
-  - travis_retry composer self-update
   - travis_retry composer install --no-interaction --no-suggest --prefer-source --dev
   - travis_retry composer require "nyholm/psr7:^1.2"