Fixup bundler version for rubies with bundler as a default gem

[ojab]

Basically use specific bundler version until lockfile is created. Fixes #117, test run which previously failed can be found here.

[eregon]

The failed log is actually https://github.com/ojab/watir-rails/runs/1500383889?check_suite_focus=true

  /opt/hostedtoolcache/Ruby/2.6.3/x64/bin/bundle lock
  Fetching gem metadata from https://rubygems.org/...........
  Fetching gem metadata from https://rubygems.org/.
  Resolving dependencies...
  Bundler could not find compatible versions for gem "bundler":
    In Gemfile.rails-4.x:
      rails (~> 4.0) was resolved to 4.2.11.3, which depends on
        bundler (>= 1.3.0, < 2.0)
  
    Current Bundler version:
      bundler (2.1.4)
  This Gemfile requires a different version of Bundler.
  Perhaps you need to update Bundler by running `gem install bundler`?
  
  Could not find gem 'bundler (>= 1.3.0, < 2.0)', which is required by gem 'rails
  (~> 4.0)', in any of the sources.
  
  Bundler could not find compatible versions for gem "rails":
    In Gemfile.rails-4.x:
      rails (~> 4.0)
  
      watir-rails was resolved to 2.2.2, which depends on
        rails (>= 3)

[eregon]

To clarify, this only affects Ruby 2.7+ which ships with Bundler 2, when there is no lockfile, and bundler: 1 is specified.

[ojab]

BUNDLER_VERSION works fine \o/ https://github.com/ojab/watir-rails/runs/1503360760?check_suite_focus=true

[eregon]

Thanks for the PR!