Update validator badge to 2.0.0, fix #265

[mcupak]

Here's a proposal that fixes our build with respect to changes in validator badge. After this, we will:

• Use the recently released tag of validator badge, i.e. no breaking changes like this in the future.
• Use quit maven output, see #265.
• Check for validation errors, ignore warnings. This is probably not ideal, but IMHO the best solution until validator badge fully supports OAS 3.