Update py-borgbackup to 1.1.6.

Version 1.1.6 (2018-06-11)
--------------------------

Compatibility notes:

- When upgrading from borg 1.0.x to 1.1.x, please note:

  - read all the compatibility notes for 1.1.0*, starting from 1.1.0b1.
  - borg upgrade: you do not need to and you also should not run it.
  - borg might ask some security-related questions once after upgrading.
    You can answer them either manually or via environment variable.
    One known case is if you use unencrypted repositories, then it will ask
    about a unknown unencrypted repository one time.
  - your first backup with 1.1.x might be significantly slower (it might
    completely read, chunk, hash a lot files) - this is due to the
    --files-cache mode change (and happens every time you change mode).
    You can avoid the one-time slowdown by using the pre-1.1.0rc4-compatible
    mode (but that is less safe for detecting changed files than the default).
    See the --files-cache docs for details.
- 1.1.6 changes:

  - also allow msgpack-python 0.5.6.

Fixes:

- fix borg exception handling on ENOSPC error with xattrs, #3808
- prune: fix/improve overall progress display
- borg config repo ... does not need cache/manifest/key, #3802
- debug dump-repo-objs should not depend on a manifest obj
- pypi package:

  - include .coveragerc, needed by tox.ini
  - fix package long description, #3854

New features:

- mount: add uid, gid, umask mount options
- delete:

  - only commit once, #3823
  - implement --dry-run, #3822
- check:

  - show progress while rebuilding missing manifest, #3787
  - more --repair output
- borg config --list <repo>, #3612

Other changes:

- update msgpack requirement, #3753
- update bundled zstd to 1.3.4, #3745
- update bundled lz4 code to 1.8.2, #3870
- docs:

  - describe what BORG_LIBZSTD_PREFIX does
  - fix and deduplicate encryption quickstart docs, #3776
- vagrant:

  - FUSE for macOS: upgrade 3.7.1 to 3.8.0
  - exclude macOS High Sierra upgrade on the darwin64 machine
  - remove borgbackup.egg-info dir in fs_init (after rsync)
  - use pyenv-based build/test on jessie32/62
  - use local 32 and 64bit debian jessie boxes
  - use "vagrant" as username for new xenial box
- travis OS X: use xcode 8.3 (not broken)


(bsiegert)