Allowing composer installations to install with `stable`

There was an issue for some people using composer that running their update command failed because phpDocumentor depended on two unstable (dev) dependencies. With earlier releases this was necessary because of crippling bugs in released versions that were fixed in their master branch.

Both packages have tagged a new version and this release of phpDocumentor fixes the composer installation issue by using the most recently released version of those dependencies.

FIXED:     #1133: Fixed subpackage without package validation
FIXED:     #1120: Set stable versions of DOMPDF and ezcDocument in Composer.json