Improvement for backwards compatibility

?P should be used to speficy a subpattern, is recommended for backwards compatibility as can be seen on official documentation http://www.php.net/manual/pt_BR/function.preg-match.php
Merge pull request #371 from D-BlindSide/patch-1

Improvement for backwards compatibility
Updated UPGRADE.md and composer.json for 0.8.0
Added minimum-stability: dev back to composer.json
Bumped maximum doctrine version to 2.6-dev
Moved minimum-stability
Merge pull request #372 from spiffyjr/release/0.8.0

Prepare for 0.8.0 release
Merge remote-tracking branch 'upstream/master'