Throw exception when using S3 Block FileSystem

[JoshRosen]

This patch extends spark-redshift to throw exceptions if the tempdir URI would create a S3 Block Filesystem. spark-redshift will only work with S3 Native FileSystems.

See #38 for more background discussion of this issue.

[codecov-io]

Current coverage is 95.04%

Merging #76 into master will increase coverage by +0.09% as of f50539f

@@            master     #76   diff @@
======================================
  Files           11      11       
  Stmts          436     444     +8
  Branches       100     104     +4
  Methods          0       0       
======================================
+ Hit            414     422     +8
  Partial          0       0       
  Missed          22      22       

Review entire Coverage Diff as of f50539f

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