Use prefer-lowest to test against dependencies.

Inspired by http://evertpot.com/testing-composer-prefer-lowest/.

- Update versions of php-vcr that we actually support
- Add Symphony YAML 2.1 explicitly to fix build errors on Travis

Also code formatting.

.travis.yml

@@ -7,10 +7,9 @@ php:
   - 5.6
 
 env:
-  - SYMFONY_VERSION="~2.3"
-  # Test against previous minor releases of Symfony 2.x to ensure that we
-  # actually support the versions we specify in our requirements.
-  - SYMFONY_VERSION="2.3.*"
+  matrix:
+    - PREFER_LOWEST="--prefer-lowest"
+    - PREFER_LOWEST=""
 
 sudo: false
 
@@ -19,9 +18,7 @@ before_script:
   # rate limits resulting in 502 HTTP responses, build errors and
   # Composer\Downloader\TransportException.
   # https://github.com/symfony/symfony/issues/4687
-  - composer install --dev --no-interaction --prefer-source
-  # Update the Symfony version to test against.
-  - composer require symfony/options-resolver:${SYMFONY_VERSION}
+  - composer update --dev --no-interaction --prefer-source $PREFER_LOWEST
 
 script:
   - mkdir -p build/logs

composer.json

@@ -23,8 +23,10 @@
     "require-dev": {
         "kasperg/phing-github": "0.2.*",
         "phing/phing": "~2.7",
-        "php-vcr/phpunit-testlistener-vcr": "1.1.*",
+        "php-vcr/php-vcr": "~1.2",
+        "php-vcr/phpunit-testlistener-vcr": "~1.1.2",
         "phpunit/phpunit": "~4.4",
-	"psr/log": "~1.0"
+        "psr/log": "~1.0",
+        "symfony/yaml": "~2.1"
     }
 }