travis: migrating to container-based infrastructure

.travis.yml

@@ -32,6 +32,11 @@ after_failure:
 
 before_script:
     # Install Nette Tester & Code Checker
-    - composer self-update
-    - composer update --no-interaction --dev --prefer-source $dependencies
-    - composer create-project nette/code-checker code-checker ~2.2 --no-interaction --prefer-source
+    - travis_retry composer update --no-interaction --prefer-dist $dependencies
+    - travis_retry composer create-project nette/code-checker code-checker ~2.5 --no-interaction
+
+sudo: false
+
+cache:
+    directories:
+        - $HOME/.composer/cache