fix tests, upgrade phpunit up to 4.3

there were errors because expression interface not found, something like this:

```
3) Liip\ImagineBundle\Tests\LiipImagineBundleTest::testAddResolversCompilerPassOnBuild
PHPUnit_Framework_MockObject_RuntimeException: Cannot mock Symfony\Component\DependencyInjection\ContainerBuilder::addExpressionLanguageProvider() because a class or interface used in the signature is not loaded

/home/vagrant/projects/Liip/LiipImagineBundleSandbox/vendor/liip/imagine-bundle/Liip/ImagineBundle/Tests/LiipImagineBundleTest.php:166
/home/vagrant/projects/Liip/LiipImagineBundleSandbox/vendor/liip/imagine-bundle/Liip/ImagineBundle/Tests/LiipImagineBundleTest.php:58

Caused by
ReflectionException: Class Symfony\Component\ExpressionLanguage\ExpressionFunctionProviderInterface does not exist

/home/vagrant/projects/Liip/LiipImagineBundleSandbox/vendor/liip/imagine-bundle/Liip/ImagineBundle/Tests/LiipImagineBundleTest.php:166
/home/vagrant/projects/Liip/LiipImagineBundleSandbox/vendor/liip/imagine-bundle/Liip/ImagineBundle/Tests/LiipImagineBundleTest.php:58
```

I did a quick fix, still have to find a better solution