add support for bundler >= 1.6.x

bundler changed their API for Bundler::UI in 1.6.0 via this commit:

  https://github.com/bundler/bundler/commit/5e38e2d7

This caused various issues, e.g.

  https://github.com/mitchellh/vagrant/issues/3451
  https://github.com/test-kitchen/test-kitchen/issues/401

which will hopefully be resolved by teaching vagrant to cope with both
API versions.
privisoners/salt: proper bootstrap URL [GH-3583]
Merge pull request #3587 from aspiers/bundler-1.6

add support for bundler >= 1.6.x
Update travis, don't need to install custom Bundler
providers/virtualbox: set proper mac address on priv net [GH-3588]
Move Windows symlink re-establishment after reboot into capability

This reduces the complexity and likelihood of provisioner bugs.
Added test to verify Windows symlinks to shares are fixed
Merge pull request #3592 from mitchellh/reboot-capability-should-fix-symlinks

guests/windows: reboot capability should fix symlinks
website/docs: docker provider
website/docs: docker commands
website/docs: rdp command
website/docs: post_up_message
website/docs: force flags on init and box remove
website/docs: run once and always provisioners
Default the WinRM shell provisioner to privileged (elevated)
Fix remote/downloaded shell scripts

The temporary shell script that we downloaded the script too was being deleted twice, thus causing an error on the secondary delete.
Merge pull request #3593 from mitchellh/elevated-winrm-shell-provisioner

provisioner/shell: Elevated winrm shell provisioner