Handle s3 source
Add specs
Update changelog
Rewrite unless case flow with positive case first.

Small change for readability as advised by PullReview. Using unless
with else makes the code more difficult to follow as the else part is a
double negative. This commit rewrites the flow with the positive case
first.
Merge pull request #3087 from nickpellant/master

Rewrite unless case flow with positive case first.
Merge pull request #3086 from tryba/s3-auth-support

bundler/bundler-features #46: S3 source support