Improve exception message

[claudusd]

Hi,

2 little change :

• One for the exception's message when no denormalizer supported.
• Travis doesn't execute test.

[henrikbjorn]

Seems it fails, can you look at it? Thanks :)

[claudusd]

One year and half later ...

Travis test are failing because when composer install with prefer-lowest option, the doctrine/comon version are 2.1.3. This version hasn't autoload configuration in the composer.json

I should set the doctrine version at 2.2 because the doctrine normalizer use the getIdentifierValues method from interface Doctrine\Common\Persistence\Mapping\ClassMetadata introduce in doctrine/common 2.2.0-RC4 (commit)