Port version_spec.rb changes from metasploit-concern
Port version_spec.rb changes from metasploit-concern

MSP-9605

metasploit-concern had an improvement to handle pull request builds on
travis-ci, so integrate it in metasploit-credential too for when
metasploit-credential is public and building on travis-ci.

rapid7/metasploit-credential,rapid7/metasploit-credential,rapid7/metasploit-credential