[JENKINS-28120]

Reflecting more code review comments from
https://github.com/jenkinsci/jenkins/pull/1679

- Jesse wants to keep the variable pointing to 1.6
- Update executable-war so that it fails gracefully if run with Java6 or
  earlier.
- CI build fails with VerifyError due to a regression in JDK.
  To support wide range of development environment, just cut off the
  verifier.
- Have enforcer require 1.7 although javac fails in a kinda obvious way.