Provide detect_encoding option for S3

With `detect_encoding: true`, the S3 provider will read each file
with the `file` utility.
If the file is determined to be gzip'd or compress'd, we set
:content_encoding option so that files can be sent compressed.

This addresses https://github.com/travis-ci/travis-ci/issues/2400
Bump Ruby dependency to 1.9.3 and up [skip ci]

We are not testing on 1.8.x any more
Bump version to 1.6.4
Document detect-encoding flag for S3 [skip ci]