Clean up Travis CI build

Author: sebastianbergmann

.travis.yml

@@ -2,11 +2,20 @@ language: php
 
 php:
     - 5.6
+    - 7.0
+    - nightly
 
-before_script:
-    - COMPOSER_ROOT_VERSION=dev-master composer install --prefer-source
+sudo: false
 
-script: vendor/bin/phpunit --configuration build
+before_install:
+    - composer self-update
+
+install:
+    - travis_retry composer install --no-interaction --prefer-source
+
+script:
+
+    - vendor/bin/phpunit --configuration build
 
 notifications:
   email: false