Validate versions for POM dependencies

I've lost more than 30 minutes during the last XWiki release because of dependencies having wrong versions so I got fed up. In order to improve our release speed we need to ensure we have valid POMs that won't generate a build issue during releases.

Example of invalid dependency definition:

    <dependency>
      <groupId>org.xwiki.rendering</groupId>
      <artifactId>xwiki-rendering-macro-html</artifactId>
      <version>${commons.version}</version>
      <scope>runtime</scope>
    </dependency>