[SPARK-1684] [PROJECT INFRA] Merge script should standardize SPARK-XXX prefix

Cleans up the pull request title in the merge script to follow conventions outlined in the wiki under Contributing Code.
https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark#ContributingtoSpark-ContributingCode

[MODULE] SPARK-XXXX: Description

Author: texasmichelle <texasmichelle@gmail.com>

Closes #5149 from texasmichelle/master and squashes the following commits:

9b6b0a7 [texasmichelle] resolved variable scope issue
7d5fa20 [texasmichelle] only prompt if title has been modified
8c195bb [texasmichelle] removed erroneous line
4f1ed46 [texasmichelle] Deque removal, logic simplifications, & prompt user to pick a title (orig or modified)
df73f6a [texasmichelle] reworked regex's to enforce brackets around JIRA ref
43b5aed [texasmichelle] Merge remote-tracking branch 'apache/master'
25229c6 [texasmichelle] Merge remote-tracking branch 'apache/master'
aa20a6e [texasmichelle] Move code into main() and add doctest for new text parsing method
48520ba [texasmichelle] SPARK-1684: Corrected import statement
042099d [texasmichelle] SPARK-1684 Merge script should standardize SPARK-XXX prefix
8f4a7d1 [texasmichelle] SPARK-1684 Merge script should standardize SPARK-XXX prefix