Remove obsolete MetasploitDataModels::Validators

MSP-10654
Remove MetasploitDataModels.require_models

It's no longer supported/needed because all loading happens with
MetasploitDataModels::Engine.
Remove obsolete MetasploitDataModels.app_pathname

MSP-10654
Remove PRERELEASE check block copied from metasploit-credential

MSP-10654
Fix PRERELEASE for branch

MSP-10654
Fix version_spec.rb to work with tag builds on travis-ci

MSP-10654
Merge branch 'bug/MSP-10654/compatibility'
removing PRERELEASE from version.rb