Add line continuation for script to work w/ py2.7.5

Error was -

$ SPARK_HOME=$PWD/dist ./dev/create-release/generate-changelist.py
  File "./dev/create-release/generate-changelist.py", line 128
    if day < SPARK_REPO_CHANGE_DATE1 or
                                      ^
SyntaxError: invalid syntax

Author: Matthew Farrellee <matt@redhat.com>

Closes #2139 from mattf/master-fix-generate-changelist.py-0 and squashes the following commits:

6b3a900 [Matthew Farrellee] Add line continuation for script to work w/ py2.7.5
(cherry picked from commit 64d8ecbbe94c47236ff2d8c94d7401636ba6fca4)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>