Adds Doctrine's mapping validation on Travis.

[phpeek]

Runs doctrine:schema:validate command with database sync skipping and
no interaction flags.

This is what I usually setup in CI for my projects to avoid common Doctrine's mapping errors e.g. missing inversedBy properties.

[javiereguiluz]

I like this a lot! Really useful. Thanks for contributing it.

[stof]

Note that for my work project, I even remove the --skip-sync option, so that forgetting to generate a DB migration (or making a mistake in it) also makes it red.

[javiereguiluz]

@stof would you recommend to remove --skip-sync here too? Thanks!

[stof]

it depends how you handle schema updates in the project.
Looking at the Travis config, it does not seem to be creating the schema from scratch itself with doctrine:schema:create or doctrine:schema:update, so it might make sense to ensure the sync too, if the update of the schema requires an action by the contributor.

[javiereguiluz]

OK. Let's merge this one as is, and when we start working with migrations, we can tweak the command. Thanks!