[Validator] ClassMetadata use deprecated methods

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: ?
Todo: Nothing
License of the code: MIT
Documentation PR: Nothing

getValue() is deprecated since version 2.2 and will be removed in 2.3. Use getPropertyValue() instead.

ClassMetadata is still using the deprecated method, changed it to getPropertyValue to prevent a trigger error.