Update version to 0.5.0 on composer.json.

And package.ini (although it need more tweeks).

On composer.json I removed the version declaration, since it is taken
from Git Tags (by Packagist) and added keywords to ease the search.

PS: I have a (big and bad) feeling this will break Travis because of
Respect/Foundation.
Skip instead of fail if Zend/Validator is not present.

It makes more sense to skip a test that can't be executed than failing
indicating a problem with the test suite.