Simplify macOS builds

[mtrmac]

No description provided.

[mtrmac]

NOTE: This is somewhat of a time bomb in that it removes the capitalized Sirupsen/logrus from vendor.conf, but not from the vendor subdirectory; on macOS having both in vendor.conf breaks.

On rm -rf vendor; vdnr:

2017/09/02 01:30:21 Errors on clone:
github.com/Sirupsen/logrus: Err: exit status 128, out: fatal: destination path '/Users/mitr/Go/src/github.com/projectatomic/skopeo/vendor/github.com/Sirupsen/logrus' already exists and is not an empty directory.

On vndr with an existing directory structure (it gets more weird):

2017/09/02 01:29:31 Errors on clone:
github.com/sirupsen/logrus: Err: exit status 128, out: Cloning into '/Users/mitr/Go/src/github.com/projectatomic/skopeo/vendor/github.com/sirupsen/logrus'...
fatal: --stdin requires a git repository
fatal: index-pack failed

github.com/Sirupsen/logrus: Err: exit status 128, out: Cloning into '/Users/mitr/Go/src/github.com/projectatomic/skopeo/vendor/github.com/Sirupsen/logrus'...
error: could not lock config file /Users/mitr/Go/src/github.com/projectatomic/skopeo/vendor/github.com/Sirupsen/logrus/.git/config: No such file or directory
fatal: could not set 'remote.origin.fetch' to '+refs/heads/*:refs/remotes/origin/*'

Here’s hoping that opencontainers/image-tools#175 / opencontainers/image-tools#176 will be done soon…

[mtrmac]

@runcom PTAL. I have dropped the partial sirupsen/Sirupsen workaround in favor of handling that separately in #426.

[runcom]

LGTM