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.
Merge branch 'tng' into rework-sorting