Merge pull request #737 from jasonthomas/master

 Update STATIC_URL on -dev to alternate S3 endpoint
Revert file storage backends on -dev
Merge pull request #738 from jasonthomas/s3

Revert file storage backends on -dev
Use the latest version of signing-clients

Corrects problems with certain signed apps
Merge pull request #739 from rtilder/master

Use the latest version of signing-clients