Fixed inversed Gd alpha
Unsupported test has been skipped for Gmagick.
Update ImageMagick download link
Merge branch 'master' into develop

* master:
  Update ImageMagick download link
Updated the extentions on travis
Use mergeImageLayers over flattenImages
Fixed another inverted gd alpha channel calculation
Merge pull request #356 from RadekDvorak/gdOpacity

Fixed inversed Gd alpha
Merge pull request #361 from GrahamCampbell/patch-1

Updated the extentions on travis