Update introduction.rst

In order to have coloring for this block too :)
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
Imagick alpha channel rounding removed
Skipped gmagick alpha channel tests
Merge pull request #341 from PunKeel/patch-1

Update introduction.rst
Add given coordinates to Point exception to ease debugging.
Fix array_merge in AbstractMetadataReader when file is not local

The parentheses were wrong for this case, leaving out the metadata
for non local files
Merge pull request #381 from javaguirre/metadata-fix

Fix array_merge in AbstractMetadataReader when file is not local
Fix image magick URL
Merge pull request #382 from romainneutron/fix-build

Fix imagemagick URL
Make the extension installation fail when any statement fails

This gives a much better feedback when there is an error in the installation
Merge pull request #383 from stof/patch-1

Make the extension installation fail when any statement fails