MAINTENANCE: Automated closing of pull requests.

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

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