Log Message:
Update ruby-bundler to 1.15.4.

## 1.15.4 (2017-08-19)

Bugfixes:

  - handle file conflicts gracefully in `bundle gem` (@rafaelfranca, @segiddins)
  - bundler will fail gracefully when the bundle path contains the system path
    separator (#5485, ajwann)
  - failed gem downloads will be retried consistently across different
    RubyGems versions (@shayonj)
  - `bundle pristine` will respect build options while re-building native
    extensions (@NickLaMuro)

## 1.15.3 (2017-07-21)

Bugfixes:

  - ensure that empty strings passed to `bundle config` are serialized &
    parsed properly (#5881, @segiddins)
  - avoid printing an outdated version warning when running a parseable
    command (@segiddins)

## 1.15.2 (2017-07-17)

Features:

  - new gemfiles created by bundler will include an explicit `github` git
    source that uses `https` (@segiddins)

Bugfixes:

  - inline gemfiles work when `BUNDLE_BIN` is set (#5847, @segiddins)
  - avoid using the old dependency API when there are no changes to the
    compact index files (#5373, @greysteil)
  - fail gracefully when the full index serves gemspecs with invalid
    dependencies (#5797, @segiddins)
  - support installing gemfiles that use `eval_gemfile`, `:path` gems with
    relative paths, and `--deployment` simultaneously (@NickLaMuro)
  - `bundle config` will print settings as the type they are interpreted as
    (@segiddins)
  - respect the `no_proxy` environment variable when making network requests
    (#5781, @jakauppila)
  - commands invoked with `--verbose` will not have default flags printed
    (@segiddins)
  - allow `bundle viz` to work when another gem has a requirable `grapviz`
    file (#5707, @segiddins)
  - ensure bundler puts activated gems on the `$LOAD_PATH` in a consistent
    order (#5696, @segiddins)