Merge tag '1.3.11' into develop

Release: 1.3.11
Preparation for new development cycle after release 1.3.11
Update Changelog.
Refactoring some code
Update travis wordpress version
Refactor the upload of a file somewhat
Update changelog
Further refactoring
Introduce library Intervention\Image\Image

Remove our home written resize functions and use a known library for doing this work.
Start using new image library
Remove quality and maker from function.

The make variable wasn't used.
Instead of having flexible quality, use same quality for each resized image.
Replace thumbnail of 400 size with 800
Move max image settings to Constants
Reformat code to PSR-2
Update changelog
Version bump 1.4.0-dev.15
Merge branch 'feature/add_image_library' into develop
Sort the use statements
Add some more files to ignore during FTP
Extract class from file.

PSR standards only allows 1 class per file.