SplFileInfo support

This adds support for creating an Image from an SplFileInfo object, in much the same way that we already support creating from a Symfony UploadFile. This will allow the user to use the Symfony Finder (which returns File objects that extend SplFileInfo), and other libraries that use or extend the SplFileInfo object.

I've left the `AbstractReader::isSymfonyUpload()` function for backwards compatibility, even though UploadFiles will be caught by the `AbstractReader::isSplFileInfo()` check (as UploadFile extends SplFileInfo). I've added an assertion in the tests that reflects this.

I've also had to update the Mockery version in composer.json to 0.9.x@dev, in order to be able to mock SplFileInfo classes properly.