POM-40. Require JDK 7 to compile Kiji projects

issue: https://jira.kiji.org/browse/POM-40
review: https://review.kiji.org/r/2007
EXP-451. Express flow steps dont always start on YARN

 - Revamp express.py

issue: https://jira.kiji.org/browse/EXP-451