Fixed generate:travis-yml command bug using Symfony Finder

The `generate:travis-yml` was looking for test files in plugins, but it was looking in `tests/*Test.php` or `tests/*/*Test.php`. And of course, for plugins that had plugins in a 3 level deep subdirectory (i.e. `tests/*/*/*Test.php`) the command was failing because no tests were found. This is the case for example for the ActivityLog plugin.

I could have fixed it by adding another test to the list (because `glob()` doesn't match recursively):

```php
$testFiles = array_merge(glob($folderPath . "/*/*Test.php"), glob($folderPath . "/*Test.php"), glob($folderPath . "/*/*/*Test.php"));
return !empty($testFiles);
```

But this is not really ideal, so instead of writing useless complex code I went with using [Symfony's Finder component](http://symfony.com/doc/current/components/finder.html):

```php
$finder = new Finder();
$finder->files()
    ->in($folderPath)
    ->name('*Test.php');
return count($finder) > 0;
```

So that's the reason I'm opening a pull request: **do you think that's fine to introduce a new dependency to that component?** I've been using the Finder for a while in other projects and it's really useful, I'm guessing it could be reused in many places in Piwik.