Fixes Object of class SplFileInfo could not be converted to boolean

When using mockery to mock a file like so:

```PHP
    protected function setUp()
    {
        $stream = new TokenStream();
        $stream->setCode(self::FILE_CONTENTS);

        /** @var Source $source */
        $source = (new Instantiator())->instantiate('TheDevNetwork\\CodeTools\\Model\\Source');
        $file = (new Instantiator())->instantiate('Symfony\\Component\\Finder\\SplFileInfo');
        $fileMock = Mockery::mock($file);
        $fileMock
            ->shouldReceive('getBaseName')
            ->atLeast(1)
            ->andReturn('VirtualClass');

        $source->setStream($stream);
        $sourceMock = Mockery::mock($source);
        $sourceMock
            ->shouldReceive('getFile')
            ->atLeast(1)
            ->andReturn($fileMock);

        $this->objectHelper = new ObjectHelper($source);
    }
```

An exception is thrown: 
Object of class Symfony\Component\Finder\SplFileInfo could not be converted to boolean

This fixes it.
Merge pull request #379 from vpassapera/patch-1

Fixes Object of class SplFileInfo could not be converted to boolean
Add dependency on doctrine/instantiator
Use doctrine instantiator where possible