Fix installation of system bundler
Fix installation of system bundler

Previously it was being installed to the :bundle_path
(`/tmp/bundled_app/.bundle`), but the `bundle` helper uses the
`system_gem_path("bin/bundle")`. That means the first `bundle install`in
the spec was actually failing, but not affecting the test status because
of not being called as `bundle!`.

mvz/bundler,bundler/bundler,mvz/bundler,bundler/bundler