Update RC2 bundler version

[philipefarias]

Keeping up-to-date with production.

Changes since last version used (1.15.1):

1.15.3 (2017-07-21)

Bugfixes:

• ensure that empty strings passed to bundle config are serialized & parsed properly (TypeError: no implicit conversion of Hash into String rubygems/bundler#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• avoid printing an outdated version warning when running a parseable command (@segiddins)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 (bundler inline fails when BUNDLE_BIN is set (default in docker) rubygems/bundler#5847, @segiddins)
• avoid using the old dependency API when there are no changes to the compact index files (Falling back to the dependency API on 1.14 rubygems/bundler#5373, @greysteil)
• fail gracefully when the full index serves gemspecs with invalid dependencies (bundler error rubygems/bundler#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 (Bundler not working with Artifactory rubygems/bundler#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 ([SpecSet] Sort by name in #tsort rubygems/bundler#5707, @segiddins)
• ensure bundler puts activated gems on the $LOAD_PATH in a consistent order (Bundler 1.15.0 having issues with omniauth gems rubygems/bundler#5696, @segiddins)