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
I generally prefer to use parentheses for this:

    if (really long condition that
      spans more than one line):

But this is fine too. Since you're here, can you fix the indentation on the next line?