MAINTENANCE: Automated closing of pull requests.

This commit exists to close the following pull requests on Github:

Closes apache#1328 (close requested by 'pwendell')
Closes apache#2314 (close requested by 'pwendell')
Closes apache#997 (close requested by 'pwendell')
Closes apache#550 (close requested by 'pwendell')
Closes apache#1506 (close requested by 'pwendell')
Closes apache#2423 (close requested by 'mengxr')
Closes apache#554 (close requested by 'joshrosen')

Author: pwendell