Remove S3Pository

S3Pository introduces race conditions in some cases. The speed benefit
of S3Pository is not apparent. This removes S3Pository, but can be
added back if a regression in build speed is introduced.
Use maven primed from maven central.
Update md5 tests with new checksum.
Merge pull request #41 from heroku/kill-s3pository

Remove S3Pository