Updated translations from transifex.
Fix another issue in UpgradeHandler.
Get EventDispatcher from argument instead of Event.
Fix #512: provide value "--none--" in mm_filteritem_checkbox again

Thanks to @emkayy
check if file exists before using getimagesize()
fix code style
Merge pull request #628 from Zeromax/fix-toolbox-fileexists

[tng] ToolboxFile should check if the file exists.
Add php 5.6 to .travis.yml
Add Contao 3.4 to allowed failures as it is in beta state.
Work around the fact that Contao\Database::getInstance() always creates an instance.

Contao\Database::getInstance() always creates an instance, even when no
driver is configured.
Therefore we must check that the Db is configured properly.
Cleanup uses in class BackendIntegration\Boot.
Fix issue #627 - Event dispatcher was not passed to DrawSetting::modelToLabelDefault().
Fix code style issues in Item.php
Refactor Item::isArrayEmpty() to reduce complexity.
Fix type cast code style errors (code style was changed).
Fix phpDocs.
Properly return css class.
Wrap long line.
Refactor DCA handler classes for tl_module and tl_content into base class.

Reducing copy paste occurences.
Add missing return.