travis: migrating to container-based infrastructure

Author: dg

.travis.yml

@@ -29,5 +29,11 @@ after_failure:
 
 before_script:
     # Install Nette Tester & Code Checker
-    - composer install --no-interaction --prefer-source
-    - composer create-project nette/code-checker temp/code-checker ~2.5 --no-interaction --prefer-source
+    - travis_retry composer install --no-interaction --prefer-dist
+    - travis_retry composer create-project nette/code-checker temp/code-checker ~2.5 --no-interaction
+  
+sudo: false
+
+cache:
+    directories:
+        - $HOME/.composer/cache