models/Build: refactor validation of url

URL fields where not validated when adding Build to CBL via scripts. I've refactored importing build logic about half year ago to use serializers that validate data. Now I cannot upgrade one instance of CBL because there builds being added with git url schema (git://). Unfortunatelly we have to change field type, becuse URLField is hardcoded only to support http/s and ftp/s schemas, and providing list of validators doesn't overide that logic.