Propel2-710 Propel compatibility with Symfony 2.5 components

added check for LegacyValidator class in order to load the correct Validator classes below SF 2.5 and from 2.5
replaced BufferedOutput with StreamOutput class which is available since SF 2.3
changed symfony requirements to ~2.3

fixes #710
Merge pull request #716 from nymo/Fix-Propel2-710-compability-with-SF-2.5

Propel2-710 Propel compatibility with Symfony 2.5 components