Uses composer command from travis-ci
Adds option to convert call so no <file>-0.ext and <file>-1.ext files are generated if the original file contains more than one image
Uses the "-flatten" option to merge several image layers so "convert" won't produce <file>-0.ext to <file>-n.ext files
Merge pull request #314 from nsendetzky/master

Uses the "-flatten" option to merge several image layers so "convert" won't produce <file>-0.ext to <file>-n.ext files