SPARK-5776 JIRA version not of form x.y.z breaks merge_spark_pr.py

Consider only x.y.z verisons from JIRA. CC JoshRosen who will probably know this script well.
Alternative is to call the version "2.0.0" after all in JIRA.

Author: Sean Owen <sowen@cloudera.com>

Closes #4570 from srowen/SPARK-5776 and squashes the following commits:

fffafde [Sean Owen] Consider only x.y.z verisons from JIRA