Update Travis to use pretty Docker-based workers

These start up much faster and the only caveat is that we can't use "sudo" (which we don't currently use anyhow).

Also, I've updated the Go version here to match what's in the Dockerfile.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6884 from tianon/use-docker-based-travis

Update Travis to use pretty Docker-based workers
Standardize "curl" arguments across the repo to be or include "-sSL"

- https://botbot.me/freenode/docker-dev/msg/17315692/
- https://botbot.me/freenode/docker-dev/msg/17315994/

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update Makefile to default BINDDIR to nothing if DOCKER_HOST is set

If "DOCKER_HOST" is set, we can usually assume the user is connecting to a remote Docker and thus not bind mount anything by default (meaning the Makefile will more often DWIM for our users).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6823 from folieadrien/move_integration_tests

Convert TestGetImagesByName into unit and integration-cli test
Merge pull request #6847 from SvenDowideit/add-os-and-arch-to-docker-version

add client OS and arch to docker version
Merge pull request #6371 from jpetazzo/6370-masquerade

Select masquerade by outgoing interface rather than by destination subne...
Merge pull request #6508 from mheon/signals

Increase size of buffer for signal forwarding with --sig-proxy
Merge pull request #6859 from LK4D4/move_buildfile_tests_to_cli

Moved last build tests to integration-cli
Merge pull request #6645 from snitm/dmthinp-devel

Dmthinp devel
Merge pull request #6452 from mheon/selinux_btrfs_fix

Error if Docker daemon starts with BTRFS graph driver and SELinux enabled
Merge pull request #6702 from mrunalp/dev/helper_fns

Move setting hostConfig and pulling images into helper functions.
Merge pull request #6850 from leeplay/patch-2

Update README.md
docker save: more integration tests

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #6821 from LK4D4/broadcast_writer_refactor

Broadcast writer refactor
Merge pull request #6799 from LK4D4/refactor_portallocator

Refactoring portallocator
We haven't required lxc and aufs for years now...

Well, maybe not years, but internet years...

Docker-DCO-1.1-Signed-off-by: Timothy Hobbs <timothyhobbs@seznam.cz> (github: timthelion)
Convert TestGetImagesJSON into several unit tests

Docker-DCO-1.1-Signed-off-by: Guilherme Salgado <gsalgado@gmail.com> (github: gsalgado)
Merge pull request #6829 from unclejack/update_archive_tar

update vendored archive/tar to lower the number of memory allocations
Merge pull request #6813 from gsalgado/convert-testgetimagesjson

Convert TestGetImagesJSON into several unit tests