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

dev/merge_spark_pr.py

@@ -244,6 +244,8 @@ def resolve_jira_issue(merge_branches, comment, default_jira_id=""):
     versions = asf_jira.project_versions("SPARK")
     versions = sorted(versions, key=lambda x: x.name, reverse=True)
     versions = filter(lambda x: x.raw['released'] is False, versions)
+    # Consider only x.y.z versions
+    versions = filter(lambda x: re.match('\d+\.\d+\.\d+', x.name), versions)
 
     default_fix_versions = map(lambda x: fix_version_from_branch(x, versions).name, merge_branches)
     for v in default_fix_versions: