Prevent serialization of job objects

https://github.com/symfony/symfony/commit/73d56f7a1a7e2aaf7cb00230af25ca208330ce6d

Job object serialization fails for unexpected reason (serialize() must
return NULL or string), I suspect one of the object descendant's
serialize method to be badly implemented. Sadly this is hardly
debuggable as the serialization tree is quite huge (more than 200 levels
of recursivity).
PIM-2869 : use new platform tag
Merge pull request #1261 from akeneo/bugfix/sf-2.3.18

[Upgrade to SF2.3.18] Prevent serialization of job objects