Removing an awk warning and not overwriting $rev var

so that the error message makes more sense.
Ripping out the modcloth-specific S3 bucket default

instead asserting that ARTIFACTS_S3_BUCKET is present.  Updated some
docs there, too.
Allowing the `--go` and `--decrypt` flags to be in either order

not least because it's totally valid to specify `--decrypt` without
specifying `--go`.
Being a bit more defensive about `--go` and `--decrypt` flags

plus removing a bogus `shift`.
Merge pull request #7 from modcloth-labs/download-artifact-touchup

Download artifact touchup